Patterning of Conjugated Polymers for Electrochromic Devices

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Patterning of Conjugated Polymers for Electrochromic Devices
ARGUN K,AVNI ANIL ( Author, Primary )
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Copyright 2004 by Avni A. Argun


To Evrim


iv ACKNOWLEDGMENTS As I look back upon the years that have led to this dissertation, I have been fortunate to have been surrounded by the he lp and influence of numerous exceptional people. First, I sincerely thank my advisor, Professor John R. Reynol ds, for his inspiring and patient guidance throughout th is enjoyable, yet challengi ng journey. He has served as a great mentor and friend from whom I have learned plenty. He allowed me to pursue the research that I have completed and kept me on the right track with his deep insight and unique enthusiasm. I wish to thank my supervisory comm ittee members Professors Kenneth B. Wagener and David B. Tanner for their guidan ce and valuable discussions throughout my graduate studies, and Professors Alexander Angerhofer and Elliot P. Douglas for their interests in serving on my committee. I extend my thanks to Professor Alan G. MacDiarmid and Dr. Nicolas J. Pinto for welcom ing me to their lab at the University of Pennsylvania to teach me the line patterning method presented in Chapter 4. I also thank the funding agencies AFOSR (F49620-03-10091) and the ARO/MURI (DAAD19-99-10316) for their financial support and Ag fa-Gevaert for donation of EDOT and PEDOT/PSS used in this work. Several coworkers and friends have had an important role during my graduate studies in Gainesville with their discussi ons and companionships. Thanks go to Dr. Pierre-Henri Aubert, Dr. A li Cirpan, Mathieu Berard, a nd Melanie Disabb for their ongoing friendship and working closely with me on several of the projects presented in


v this dissertation. I would like to thank Be n Reeves and Christophe Grenier for the synthesis of PXDOTs and their contributions in collecting some of the data presented in Chapter 6. Other members of the Reynolds Group who deserve acknowledgement include Dr. Mohamed Bouguettaya, Dr. Said Sadki, Dr . Irina Schwendeman, Dr. Gursel Sonmez, Barry Thompson, and Nisha Ananthakrishnan for being helpful when I needed it. I also thank Maria Nikolou, my collaborator from Dr. Tanner’s Group in Physics, for enjoyable discussions. My time here would not have been the same without the social diversions provided by all my friends in Gainesville. I am particularly thankful to Enes Calik and Omer Ayyer for their continuous friendshi p and Sertac Ozcan for his pool parties. For their contributions to my interest in polymer science, I thank my undergraduate advisor Prof. efik Süzer at Bilkent University and Prof. Levent Toppare at Middle East Technical University. They taught me valuab le life lessons and prepared me well for graduate school. I also thank Emrah Ozen soy who has been my best friend during undergraduate years. I thank my parents Ay e and Hasan for allowing me to make my own decisions since I was a little child and preparing me to tack le life wherever it may take me. It is not easy to send a child away from home when he is only 10 years old and expect him to endure the complexities of life. I thank my sister Seher for being a fine example to me since the day she taught me how to read. I also wish to thank Aunt adiye for her moral support during my four years in Ankara. Finally, I give my special thanks to my wife, Evrim, for her true love and support no matter how unbearable I get. She is the source of my inspiration and my ultimate “life improvement.”


vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES.............................................................................................................x ABSTRACT....................................................................................................................... xv CHAPTER 1 INTRODUCTION........................................................................................................1 Conducting Polymers....................................................................................................1 Electrochromism of Materials......................................................................................2 Fundamentals of Electrochromism...............................................................................3 Electrochromic Contrast........................................................................................4 Coloration Efficiency............................................................................................5 Switching Speed....................................................................................................5 Stability..................................................................................................................6 Optical Memory.....................................................................................................6 The Origin of Electrochromism in Conjugated Polymers............................................7 Characterization of Electrochromic Polymers – Methods............................................9 Multi-Color Electrochromic Polymers – Color Control.............................................12 Polymer Electrochromic Devices...............................................................................17 Absorption/Transmission ECDs..........................................................................18 Reflective ECDs..................................................................................................21 ECD Applications................................................................................................22 General Patterning Methods.......................................................................................25 Optical Lithography.............................................................................................26 Electron Beam (e-beam) Lithography.................................................................26 Scanning Probe Lithography...............................................................................27 Microcontact Printing ( CP)...............................................................................28 Inkjet Printing......................................................................................................29 Patterning of ECDs.....................................................................................................30 Metal-Vapor Deposition......................................................................................30 Line Patterning....................................................................................................31 Screen Printing....................................................................................................32 Structure of Dissertation.............................................................................................32


vii 2 EXPERIMENTAL METHODS.................................................................................34 Chemicals and Materials.............................................................................................34 Preparation of Electrodes............................................................................................35 Metal Vapor Deposition......................................................................................35 Line Patterning....................................................................................................36 Electroless Metal Plating.....................................................................................37 All-Polymer Electrodes.......................................................................................38 Conductivity Measurements.......................................................................................39 Electrochromic Polymer Deposition...........................................................................40 Electrochemical Polymerization..........................................................................40 Spray Coating......................................................................................................41 Device Construction...................................................................................................42 Electrochemical Methods...........................................................................................44 Cyclic Voltammetry............................................................................................44 Chronocoulometry...............................................................................................45 Optical Methods..........................................................................................................46 Reflectance Spectroscopy....................................................................................46 Spectroelectrochemistry......................................................................................47 Single Wavelength Transient Absorption...........................................................47 Colorimetry..........................................................................................................48 3 PATTERNING OF REFLECTIVE ECDS USING SHADOW MASKS..................49 Electrode Patterning....................................................................................................51 Reflective ECDs from Microporous Gold Electrodes................................................52 Device Design and Construction.........................................................................53 Spectroelectrochemical Characterization............................................................54 Electrochromic Switching and Stability..............................................................57 Composite Coloration Efficiency (CCE).............................................................59 Open Circuit Memory..........................................................................................61 Energy Consumption...........................................................................................62 Pixelated Lateral ECDs.......................................................................................64 Reflective ECDs from Micr oporous Nickel Electrodes.............................................65 Back-Side Electrical Cont acts for Patterned ECDs....................................................67 Electrode Preparation..........................................................................................69 Reflective ECDs..................................................................................................71 Digit-Display ECD..............................................................................................74 Conclusions.................................................................................................................76 4 LINE PATTERNING OF METALLIC ELECTRODES FOR LATERAL ECDS....77 Preparation of Patterned Electrodes............................................................................78 Lateral ECDs Using Interdigitated Electrodes (IDEs)................................................80 PEDOT-PBEDOT-Cz Lateral ECDs...................................................................82 Lateral ECDs with Varying IDE Spacing...........................................................85 Other Applications of Line Patterning........................................................................89


viii 5 ALL ORGANIC ECDS..............................................................................................92 Line Patterned PEDOT/PSS Electrodes.....................................................................94 PEDOT Deposition..............................................................................................95 PBEDOT-Cz Deposition.....................................................................................99 Highly Conducting PEDOT/PSS Electrodes............................................................100 EC Polymers on PEDOT-HAPSS Electrodes...................................................102 All Organic Electrochromic Devices.................................................................106 Absorptive/transmissive ECDs .................................................................107 Dual-colored ECDs....................................................................................111 Conclusions...............................................................................................................112 6 ECDS BASED ON PROCESSABL E DIOXYTHIOPHENE POLYMERS............114 Spray Coated Electrochromic Polymer Films..........................................................116 Optoelectronic Characterization........................................................................117 Thickness Dependence of PProDOT-(EtHx)2 Films.........................................123 Coloration Efficiency........................................................................................124 Electrochromic Devices............................................................................................125 Absorptive/Transmissive ECDs........................................................................126 Reflective ECDs................................................................................................133 Conclusions...............................................................................................................137 Overall Summary and Perspective............................................................................138 LIST OF REFERENCES.................................................................................................142 BIOGRAPHICAL SKETCH...........................................................................................151


ix LIST OF TABLES Table page 1-1 Patterning methods, the highest reso lution values achieved from these methods, and their brief description...........................................................................27 3-1 Components used in construction of the reflective electrochromic devices..............53 3-2 Optical reflectivity contrast in the visible ( %RVIS) and the NIR range ( %RNIR) for the devices D1-D5.....................................................................56 3-3 Energy consumption data for D1 and D5 type devices.............................................64 3-4 Metal candidates to be used in reflective ECD applications....................................66 5-1 S urface resistance (Rs) and surface resistivity ( s) values of PEDOT/PSS coated films.......................................................................................................................... ..95 5-2 Conductivity enhancement of PEDOT/PSS using additives....................................102 5-3 Surface resistivity values of PEDOT-HAPSS........................................................103 5-4 Coloration efficiency values of a PProDOT-(Me)2/PBEDOT-NMeCz device........110 6-1 Peaks (nm) and Optical Band-gaps (eV) from the UV-Vis spectroscopy of PProDOT derivatives...........................................................................................119 6-2 Electrochromic properties of spray cast films..........................................................123 6-3 Optical and electroche mical data for coloration efficiency measurements.............132


x LIST OF FIGURES Figure page 1-1 Doping mechanism for PProDOT: (a) Ne utral form, (b) Slightly doped radical cation, (c) Fully doped dication....................................................................................2 1-2 Spectroelectrochemistry of a PProDOT-(Et)2 film on ITO/Glass at applied potentials between (a) -0.1V and (o) +0.9V vs. Ag/Ag+ with 50 mV increments . .......................8 1-3 Representative electrochromic polymers. Color swatches are representations of thin films based on measured CIE 1931 Yxy color coordinates.......................................14 2-1 Schematic representation of high vacuum metal vapor deposition process.............36 2-2 Line patterning of plastic substrates: (a) PEDOT-PSS elect rodes, (b) Electroless gold deposition...................................................................................................................37 2-3 (a) Surface resistivity measurement of a thin film and (b) Four-probe conductivity measuremen t setup.....................................................................................................39 2-4 Potentiodynamic deposition of PProDOT-(Hx)2 on Pt button electrode (Electrode area = 0.02 cm2)..........................................................................................................41 2-5 (a) Schematic representation of an ab sorption/transmissive type device. (b) A reflective device scheme using porous electrodes......................................................43 2-6 Chronocoulometry experiment of a PProDOT-(EtHx)2 film on ITO: (a) The potential step, (b) Current and charge cu rves as a function of time..........................................46 2-7 Integrating sphere used for reflectiv e characterization of surface active ECDs......47 3-1 (a) Schematic representation of a reflec tive type electrochromic device (ECD) using a porous membrane electrode and (b) Cross section of the ECD...............................50 3-2 (a) Two gold pixels patterned on a polycarbonate membrane, (b) A 2 x 2 gold pattern, (c) Magnification (80x) of the metallized membrane, and (d) Image of the pattern on the glass backing plate...............................................................................52 3-3 (a) Reflectivity contrast ( R = %Rneutral %Roxidized) spectra of D2 PEDOT (A), D3 PProDOT (B), and D5 PProDOT-(Me)2 (C) devices and (b ) The two photographs represent (left) the oxidized and (right) the neutral appear ance of the active layer...55


xi 3-4 Spectroelectrochemistry of a PProDOT-(Me)2 active layer in a D5-inert type reflective device: (a) –0. 8V, (b) –0.6V, (c) –0.4V, (d) –0.2V, (e) 0.0V, (f) +0.2V, and (g) +0.4V.............................................................................................................57 3-5 (a) Temporal change in %R (1540 nm) dur ing electrochromic switching of a D3 type reflective device between –1V and +1V ev ery 1 second and (b) A single transition illustrating the switching time of the same device (-1V to +1V, =558 nm).............58 3-6 Long-term switching stability of a D5-ine rt type device switching between –1V and +1V every 3 seconds..................................................................................................59 3-7 Open circuit memory of a D5-inert type device monitored by single-wavelength reflectance spectroscopy. (a) Visible memo ry at 558 nm and (b) NIR memory at 1540 nm......................................................................................................................62 3-8 (a) Photographs of EC switc hing of PEDOT and PBEDOT-B(OR)2 on a 2 x 2 pixel gold/membrane electrode. (b) A 2 x 2 pixels device using th e patterned electrodes described above..........................................................................................................65 3-9 a) Accumulative deposition of PEDOT on a nickel coated microporous polycarbonate membrane (Electrode area = 1.7 cm2). (b) EC switching of a PEDOT device comprising nickel electrode s. Left: -1.0V, right: +1.0V.................................67 3-10 (a) An ion track etched membrane with well-defined pores (left) and a fiber-like porous membrane (right). (b) Reflective optical micrograph of a track-etched membrane. (c) Reflective optical micr ograph of a laboratory filter paper...............70 3-11 A reflective type ECD scheme us ing back-site addressed electrodes. iTransparent window, iiPProDOT-(Me)2, iiiAu, ivPorous membrane, vBack-side contact, viAn porous separator, vii Polymer counter electrode, viiiAu/plastic...............72 3-12 In-situ reflectance spectroel ectrochemistry of a PProDOT-(Me)2 ECD. Applied voltages: (a) -1.0V, (b)-0.8V, (c) -0.6V, (d) -0.2V, (e) 0 V, (f) 0.2V, (g) 0.4V, (h) 0.7V, and (i) 1.0V.....................................................................................................73 3-13 Machine-cut masks used to pattern gold on front (a) and back (b) sides of porous membranes. c) Photograph of a 7-pixe l electrochromic numeric display device showing the number “5”. De vice dimensions: 3cm x 5cm......................................75 4-1 Preparation of line patterned, gold elect rodes. (a) Computer generated designs (negative patterns), (b) Phot ographs of an interdigitate d electrode (IDE) and a 3x3 pixels pattern, and (c) Reflective opti cal micrographs of the electrodes...................79 4-2 Optical micrograph of a 100x magnified li ne patterned gold substrate to show the resolution limit is down to 30 m..............................................................................80 4-3 “Color averaging” in la teral type ECDs. (a) Electroc hemical deposition of polymer films, (b) EC switching of the resulting ECD............................................................81


xii 4-4 (a) Arrangement of polymers for lateral type ECDs shown with their photographs on gold slides. (b) Potentiodynamic deposition of PBEDOT-Cz on the IDE. (c) Charge matching of polymers. (d) EC switchi ng of two complementary polymers..............83 4-5 Electrochemical switching of a PE DOT/PBEDOT-Cz device with PBEDOT-Cz being the working electrode. (a) Multi-voltage sweep of the device between -0.5V and +1.2V. (b) Chronoamperometry and chronocolulometry of the device..............84 4-6 Negative computer images of ID Es with varying finger widths..............................86 4-7 Multi-sweep CV electropolymerization of (a) PProDOT-(Me)2 and (b) PBEDOT-Cz from their monomer electrolyte solutions onto a 2-lane IDE.....................................86 4-8 Voltage sweep of 2-lane (black), 4-la ne (red), and 6-lane (green) lateral ECDs comprising PProDOT-(Me)2 (working electrode) a nd PBEDOT-Cz (counter electrode) as the complementary colored polymer pair.............................................87 4-9 (a) The %R changes of the 2-lane, 4-lane, and 6-lane devices as a function of time as they are switched from -1.0V to +0.8V. (b) Switching time to reach the 85% of the full contrast as a function of the dist ance between the anode and the cathode..........88 4-10 EC switching between an absorptive blue state (-1.0V, left) a nd a reflective state (+0.8V, right)............................................................................................................89 4-11 EC switching of a cross patterned PEDOT device to yield high contrast (left, -1.0V) and no contrast (right, -0.2V) surfaces.....................................................................90 4-12 EC switching of PEDOT on line patterne d, interdigitated ITO/Plastic electrodes..91 5-1 Chemical structure of PEDOT/PSS..........................................................................93 5-2 Schematic representation of a PEDOT/PSS (Baytron P) coated, interdigitated plastic electrode.....................................................................................................................9 5 5-3 %Transmittance of PEDOT/PSS coated substrates vs. air.......................................96 5-4 Optical microscope pictures of EC PEDOT film on PEDOT/PSS: (a) EC PEDOT film deposited between micro-printed lin es, (b) EC PEDOT – PEDOT/PSS interface at the meniscus, (c) Magnification of the interface to show the short........................97 5-5 Redox switching of EC PEDOT betw een (–1.1V) and (+1.1V) vs. Ag/Ag+..........98 5-6 EC PEDOT deposited electrode: (a) Electrochromic switching of the PEDOT between its redox states. (b) Optical micr ograph of PEDOT deposited (middle line) and non-deposited lines..............................................................................................99 5-7 Electrochromic switching of PBEDOT-C z in TBAP (0.1M) /ACN electrolyte solution.....................................................................................................................10 0


xiii 5-8 %T of the PEDOT-HAPSS coated transparency film electrodes in the visible region with (i) one layer, (ii) two la yers, and (iii) three layers............................................103 5-9 (a) Accumulative synthesis of PEDOT fr om its monomer solution at 25 mV/s. (b) Redox charge (Qred) as a function of deposition charge (Qdep) for PEDOT. (c) CV of a PEDOT film at 10 mV/s. (d) %T change of the PEDOT film...............................105 5-10 Photographs of EC polymers on PE DOT-HAPSS electrode s in colored and bleached states. Left: Oxidized, Righ t: Neutral. Electrode areas ~ 2 cm2. (a) PEDOT, (b) PBEDOT-B(OC12)2, and (c) PBEDOT-NMeCz................................106 5-11 Schematic representation of the tran smissive/absorptive type ECD constructed from all-polymer components................................................................................106 5-12 Optical characterization of a comp lementary colored ECD using PProDOT-(Me)2 and PBEDOT-N-MeCz along with the photogra phs taken at two extreme states of the device, namely, colored and bleached..............................................................108 5-13 Representation of the color change of the PProDOT-(Me)2/PBEDOT-NMeCz device on the CIE 1931 xy chromaticity diagram..................................................110 5-14 Optical characterization of a twocolored ECD using PEDOT and PBEDOTB(OC12)2 as the EC polymers: (a) Spectroel ectrochemistry of the device. (b) Voltage dependence of percent relative luminance................................................112 6-1 Chemical structures of solution processable PProDOT-R2 polymers....................117 6-2 Photograph of spray cast films of PProDOT-(CH2OEtHx)2 (red, left) and PProDOT(C18)2 (purple, right) from 0.6% w/w toluene solutions..........................................118 6-3 Cyclic voltammograms and UV-Vi s spectra of polymers PProDOT-(Hx)2 (a) and (c) and PProDOT-(EtHx)2 (b) and (d). .........................................................................119 6-4 Three dimensional surface of the spectroelectrochemistr y of a previously switched spray cast film of PProDOT-(Hx)2 on an ITO coated glass slide............................121 6-5 (a) Relative luminance cha nge (%Y) of spray cast PProDOT-R2 films. (b) %Y vs. applied potential superimposed on cyclic voltammetry for PProDOT-(EtHx)2.......121 6-6 Thickness dependence of electrochemical and electrochromic properties of spray cast films of PProDOT-(EtHx)2...............................................................................124 6-7 Slow coloration efficiency and percent transmittance as a function of passed charge for a 150 nm film of PProDOT-(CH2OEtHx)2.........................................................125 6-8 Schematic representation of an absorptive/transmissive type PProDOT-R2/PBEDOTNMeCz device..........................................................................................................127


xiv 6-9 Voltage dependence of the relative luminance of a PProDOT-(CH2OEtHx)2 /PBEDOT-NMeCz device and photographs of the device in the bleached and dark states......................................................................................................................... 129 6-10 (a) Spectroelectrochemistry of a PProDOT-(CH2OEtHex)2/PBEDOT-NMeCz device as a function of applied voltage. (b) Spectra from the two extreme states of the device................................................................................................................130 6-11 Chronoabsorptometry (solid line) an d chronocoulometry (dashed line) for a PProDOT-(CH2OEtHx)2/PBEDOT-NMeCz electrochromic device along with the slow coloration efficiency......................................................................................133 6-12 (a) Schematic device structure of a refl ective ECD. (b) Photographs exhibiting the neutral and the oxidized appear ance on a gold reflective surface..........................135 6-13 Spectroelectrochemistry of PProDOT-(CH2OEtHex)2 containing reflective device as a function of applied voltage..............................................................................135 6-14 Electrochromic switching as the voltage of a PProDOT-(CH2OEtHex)2 containing reflective device is stepped between (a) -1.0 V and 0.0 V, (b) -0.8 V and -0.02 V, and (c) +1.2 V and + 0.05 V...................................................................................137


xv Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PATTERNING OF CONJUGATED POLYMERS FOR ELECTROCHROMIC DEVICES By Avni A. Argun December 2004 Chair: Professor John R. Reynolds Major Department: Chemistry This work details the electrical and op tical properties of electrochromic devices (ECDs) based on conjugated polymers, along with the application of a number of patterning techniques to prepare electrode s for ECDs. The use of highly porous metallized membranes in patterned reflective ECDs is introduced which allows fast electrochromic switching of dioxythiophene based polymers (5-10 Hz) with outstanding power efficiencies and long-term st abilities (180,000 switc hes). Reflectance spectroscopic characterization of these devices is performed to probe the attenuation of visible and NIR light from the metal electr ode induced by the electroactive polymer. Using metallized porous electrode s in reflective type ECDs, refl ectance contrast values of up to 90% in the NIR and ~60% in the vi sible regions are obtained. A method is developed to prepare patterned electrodes on porous substrat es where the contacts to address these electrodes are hi dden on the back of the substrates. This method permits


xvi increased density and more design flexibility for display type devices as compared to conventional front-side contact techniques. One of the greatest challenges in patterning of electronic devices is the complexity of the process to obtain finely structured electrodes. Line pa tterning, a simple and effective technique w ith high resolution (~30 m), is employed to build laterally configured reflective ECDs based upon the co lor mixing of two complementary colored polymers deposited on a patterned metal surface. Line patterned, interdigitated ECDs with varying anode-cathode spacing are assemb led to demonstrate the effect of device geometry on the switching performance. Truly all-organic ECDs are demonstrat ed for the first time by replacing conventional ITO electrodes with a highl y conducting polymer PEDOT/PSS. These ECDs comprise a complementary colored polymer pair sandwiched between two PEDOT/PSS coated plastic electrodes. An absorption/transmission and a dual-colored ECD are designed, built, and characterized to show the compatibility of PEDOT/PSS as the electrode material. Solution processability of conjugated polymer s is an important factor for large area applications. A family of alkyl and al koxy substituted organic soluble PProDOTs (PProDOT-R2) are spray cast onto ITO and gold el ectrodes and highly homogenous films with thicknesses controlled from 30-300 nm, with surface roughness values of 10-25 nm are attained. Absorption/transmi ssion ECDs built from PProDOT-(CH2OEtHx)2 yield fast switching (0.3 sec) and high opti cal contrast (77% relative luminance contrast) with coloration efficiency values of 3,800 cm2/C, by far the highest reported to date.


1 CHAPTER 1 INTRODUCTION Conducting Polymers Conducting polymers have been known sin ce 1862 when the first electrochemical synthesis of poly(aniline) (PANI) yielded a black powdery deposit.1 The interest was renewed later in 1977 through the discovery of the metallic properties of polyacetylene (PAc) by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid2-4 which led to a Nobel Prize in Chemistry in 2000. Since then, c onducting (conjugated) polymers have been investigated for their semiconducting and el ectrochemical properties resulting in a number of device applicati ons such as light-emitting diodes, electrochromics, photovoltaics, sensors, and fi eld-effect transistors. PAc is the simplest form of a c onducting polymer that has a conjugated system extending over the polymer chain. Its electr ical conductivity exhibi ts a 12 order of magnitude change when doped with iodine.2 Due to its intractability and air sensitivity, other conjugated systems derived from PAc with aromatic structures and heteroatoms were developed such as PANI, poly(thi ophene) (PTh), and poly(pyrrole) PPy. They possess high conductivity values of 101-105 S/cm when they are redox doped. The magnitude of the conductivity change depends on the doping level which can be controlled by the applied potential in th e case of electrochemical doping. The doping mechanism of a fully conjugated poly(3,4propylenedioxythiophene) (PProDOT) is given in Figure 1-1. Neutral polymer (a) is a typica l semiconductor and it exhibits an aromatic form with alternating double bonds. After removal of an electron from the polymer chain


2 (oxidative doping), a radical cat ion (polaron) is generated , and the polymer assumes a quinodial state that facilita tes charge transfer along the backbone. To maintain electroneutrality, anions diffuse into the polymer film. With in creasing doping levels, more than one electron can be removed from the chain which results in formation of a dication (bipolaron). S OO S OO S OO S OO S OO S OO S OO S OO S OO +.+ +-e--e-AAA(a) (b) (c) Figure 1-1. Doping mechanism for PProDOT . (a) Neutral form, (b) Slightly doped radical cation, (c) Fully doped dication. Electrochromism of Materials Electrochromism is broadly defined as a re versible optical change in a material induced by an external voltage, with ma ny inorganic and organic species showing electrochromism throughout th e electromagnetic spectrum.5 Suggested theoretically by J.R. Platt6 in 1961, the first examples of electrochromic mate rials and devices were demonstrated by Deb7, 8 when he started to work on amorphous and crystalline metal oxides at Cyanamid Corp. Among electrochromic (EC) materials, transition metal oxides, especially the high band gap semiconductor tungsten oxide, WO3, have received extensive attention over the past 30 years.9-11 Thin films of amorphous or polycrystalline WO3 can be prepared by vacuum evaporati on, reactive sputtering, and sol-gel methods. Initially transparent in the visible re gion, cation intercala tion (reduction) of WO3 to MxWO3 (M can be hydrogen or an alkali metal) leads to strong absorption bands in the visible region, making it a cathodically colori ng material. Many othe r inorganic materials


3 have been studied for their electrochromic prope rties such as Prussian blue, oxides of V, Mo, Nb, and Ti (cathodically coloring), and oxides of Ni, C o, and Ir (anodically coloring).12 Other EC materials include organic small molecules, such as the bipyridiliums (viologens), which are a class of materials th at are transparent in the stable dicationic state. Upon one-electron reduction, a highly co lored and exceptionally stable radical cation is formed. Thin film electrochro mism is observed for polyviologens and Nsubstituted viologens such as heptyl viologen.13 More recently, composite systems, where organic molecules are adsorbed on mesopor ous nanoparticles of doped metal oxides, have shown improved electrochromic properties.14, 15 Conjugated polymers are a third class of EC materials that have gained popularity due to their ease of processa bility, rapid response times, hi gh optical contrasts, and the ability to modify their structure to create multi-color electrochromes. Of the conjugated EC polymers, derivatives of PTh, PPy, and PANI are widely studied.16 The mechanism of the EC effect and color control will be discus sed in detail for conjugated polymers later. Conjugated polymers, while not as developed as the other systems, promise high contrast ratios, rapid response times, and long lifetimes for use in EC display technology. The ability to physically structure polymer-based electrochromic devices (ECDs) and exert control over their EC res ponses are addressed in the following sections. Fundamentals of Electrochromism There are three main types of electro chromic materials in terms of their electronically accessible optical states. The first type includes materials with at least one colored and one bleached state. These materials are especially useful for absorption/transmission type device applica tions such as smart windows and optical


4 shutters. Typical examples of this area are metal oxides, viologens, and polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT). A second class of material consists of electrochromes with two distinctive colo red states. These EC materials lack a transmissive state but are useful for display type applications wher e different colors are desired in different redox states. Polythiophe ne is a good example for this type where the thin films of this polymer switch from red to blue upon oxidation. A third class includes the growing interest in the electrochromic field where more than two color states are accessible depending on the redox state of the ma terial. This is the area where conjugated polymers have found the most interest due to their versatility for making blends, laminates, and copolymers. Additionally, th ere are inherently multi-color EC polymers such as PANI or poly(3,4-propylenedioxypyrrole ) (PProDOP). These will be discussed in detail later in the “Color C ontrol” section. Before switching to a more detailed discussion of polymer electrochromism, some of the important parameters in identifying and characterizing electrochromic materials are outlined. Electrochromic Contrast Electrochromic contrast is probably the mo st important factor in evaluating an electrochromic material. It is often reported as a percent transmittance change ( %T) at a specified wavelength where the electrochromic material has the highest optical contrast. For some applications, it is more useful to report a contrast over a specified range rather than a single wavelength. In or der to obtain an overall elect rochromic contrast, measuring the relative luminance change provides more realistic contrast values since it offers a perspective on the transmissivity of a material as it relates to the human eye perception of


5 transmittance over the entire visible spectrum.17, 18 The light source used is calibrated taking into account the sensit ivity of the human eye to different wavelengths. Coloration Efficiency The coloration efficiency (also referred to as electrochromic efficiency) is a practical tool to measure the power require ments of an electrochromic material. In essence, it determines the amount of optical density change ( OD) induced as a function of the injected/ejected electronic charge (Qd), i.e . the amount of charge necessary to produce the optical change. It is given by the equation: = OD / Qd = log Tb / Tc / Qd where (cm2/C) is the coloration efficiency at a given , and Tb and Tc are the bleached and colored transmittance values, respectively. The relationship between and the charge injected to the EC material can be us ed to evaluate the reac tion coordinate of the coloration process or the values can be reported at a sp ecific degree of coloration for practical purposes. Switching Speed Switching speed is often reported as the time required for the coloring/bleaching process of an EC material. It is important especially for applications such as dynamic displays and switchable mirrors. The switchi ng speed of electrochromic materials is dependent on several factors such as ionic c onductivity of the electr olyte, accessibility of the ions to the electroactive sites (ion diffusion in thin f ilms), magnitude of the applied potential, film thickness, and the morphology of the thin film. Today, subsecond switching rates are easily attained using polymers and composites containing small organic electrochromes.


6 Stability Electrochromic stability is usually associat ed with electrochemical stability since the degradation of the active redox couple results in the loss of electrochromic contrast and hence the performance of the EC material. Common degradation paths include irreversible oxidation or reduc tion at extreme potentials, iR loss of the electrode or the electrolyte leading to internal heating, side reactions due to the presence of water or oxygen in the cell, and the heat released due to the resistiv e parts in the system. Although current reports include switc hing stabilities of up to 106 cycles without significance performance loss, the lack of durability (e specially compared to LCDs) is still an important drawback for commercialization of EC Ds. Defect-free processing of thin films, careful charge balance of the electroactive components, and air-free sealing of devices are important factors for l ong-term operation of ECDs. Optical Memory One of the benefits of using an electrochro mic material in a display as opposed to a light emitting material is its optical memory (also called open-circuit memory) which is defined as the time an electrochromic materi al retains its absorption state after removing the electric field. In solution based electrochro mic systems such as viologens, the colored state quickly bleaches upon termination of current due to the diffusion of soluble electrochromes away from the electrodes a nd reacting in the elec trolyte (a phenomenon called self-erasing). In solid state ECDs where the electrochromes are adhered to electrodes, electrochromic memory can be as long as days or weeks with no further current required. In reality however, ECDs may require smal l refreshing charges in order to maintain the charge state because side re actions or short circuits change the desired color.


7 The Origin of Electrochromism in Conjugated Polymers Conjugated polymers such as derivati ves of PPy, PTh, and PANI display electrochromism in thin film form. Alkoxy substituted PTh derivatives, such as PEDOT have been investigated due to their ease of synthesis, high chemical stabilities in the oxidatively doped state, a nd high optical contrast va lues between redox states.19 Electrochromism in conjugated polymers occurs through changes in the conjugated polymer’s electronic character accompanied by re versible insertion and extraction of ions through the polymer film upon electro chemical oxidation and reduction. In their neutral (insulating) states, these polymers show semiconduc ting behavior with an energy gap (Eg) between the valence band (HOMO) and the conduction band (LUMO). Upon electrochemical or chemical doping (“ p-doping” for oxidation and “n-doping” for reduction), the band structure of the neutral polymer is modi fied generating lower energy intra-band transitions and crea tion of charged carriers (polarons and bipolarons), which are responsible for increased c onductivity and optical modulation. The doping process, and the resultant opti cal changes in conjugated polymers, are vividly illustrated through sp ectroelectrochemical experiments such as the one shown in Figure 1-2 for a thin film of poly( 3,3-diethyl-3,4-dihydro-2H-thieno[3,4b][1,4]dioxepine) (PProDOT-(Et)2). This polymer is purple-blue in the neutral state, and upon electrochemical oxidation switches to a transmissive sky blue in the oxidized (conducting) state . The neutral (colored) state of PProDOT-(Et)2 has a strong * absorption in the visible region and a band gap of 1.7 eV ( max = 580 nm).20 Initial oxidation (p-doping), results in a new absorpti on band in the near IR region (~ 900 nm), forming at the expense of the * transition, and is attri buted to polarons (radical


8 cations) generated along the polymer chain. Upon complete electrochemical oxidation, the * transition and the polaron absorption are fully depleted, while a lower energy transition, peaked in the NIR beyond the range of the spectrophotometer, increases. This absorption is assigned to the bi polaronic (dication) state of the conjugated polymer. Such optical and structural changes are reversib le through repeated doping and dedoping over many redox cycles, making EC polymers poten tially useful in applications for modulating transmissivity and color. Figure 1-2. Spectroelectroche mistry of a PProDOT-(Et)2 film on ITO/Glass at applied potentials between (a) -0.1V and (o) +0.9V vs. Ag/Ag+ with 50 mV increments.20 Inset shows photographs of the polymer film in its doped and neutral states. Below the photographs are shown the CIE 1931 Yxy color swatches of the corresponding states measured by in situ colorimetry .


9 Characterization of Electr ochromic Polymers–Methods To gain a deeper understanding of the el ectrochromic processes in conjugated polymers, multiple characterization methods ha ve been developed. As discussed in the previous section, spectroelectrochemistr y has been commonly used to study the electrochromic processes in conjugated polymers. However, spectroelectrochemistry does not allow one to precisely define cont rast ratios or switching speeds. Thus, our group and others have developed several other methods such as In-Situ Colorimetric Analysis,21, 22 Reflectance Analysis,23-26 Composite Coloration Efficiency,27 Slow Coloration Efficiency,28 and fast electrochrom ic switching experiments29, 30 in addition to spectroelectrochemistry. Using these primar y techniques, one can learn much about electrochromism in conjugated polymers. For any commercial electrochromic material , specific and reproducible color states and contrast ratios are require d. Therefore, In-Situ Colorimetric Analysis is used as a means of precisely defining color and contra st ratios in electrochromic polymers. The colorimetric analysis experiment is based on a set of color coordinates, such as the CIE 1931 Yxy color space.17 In this color space, Y corresponds to the brightness or luminance of a color (specifically the brightness of the transmitted light in a transmission experiment), whereas the xy coordinate of a color defines its hue and saturation. The benefit of defining a color via colorimetry rather than by simply stating a max is that the CIE system of colorimetry is based on a sta ndard observer and thus takes into account the manner in which the human eye perceives colo r. Colorimetric Analysis thus gives a precise and accurate description of color.


10 In a typical experiment, the light transmitted through a polymer film is analyzed by a colorimeter (e.g. Minolta CS 100), which yiel ds Yxy values. Perhaps the most useful information found through colorimetric analysis is the relative luminance (%Y). Here the measured luminance value (Y) is taken re lative to a standard white illuminant. Calculating the difference between %Y values measured at various applied potentials yields a measure of the contrast ratio that takes into account all wavelengths of the visible spectrum and the non-linear response of the human eye. Reflectance Analysis, where the absorbance of an EC polymer is measured through reflected light, provides useful information for investigating the optical properties of thin films on reflective substrates such as gold, platinum, and ITO. In particular, diffuse reflectance data may provide valuable inform ation about the surface topology of a film since it takes into account the scattered light in addition to the angular (specular) reflected light. In-situ reflection spect roelectroscopy methods, where the absorbance of an EC polymer is monitored at different oxidation st ates, have been used to characterize PANI, PEDOT, and PProDOT polymers.21, 23, 31 Recently, our research group has developed Composite Coloration Efficiency (CCE) to characterize the effici ency of electrochromic polymers.27 CCE is a measure of the change in optical de nsity of a material at max relative to the total amount of injected/ejected charge. CCE is thus a meas ure of how much charge is required for bleaching or coloration in an EC materi al. The experiment is based on a tandem chronoabsorptometry/chronocoulometry me thod in which the transmission at max is monitored along with the charge passed as a polymer film is switched between redox states. In a standard experiment, we calcu late CCE at 95% of the maximum optical


11 contrast. Once this 95% change is reached, litt le additional color change is perceivable to the naked eye and the complicat ions of indefinitely incr easing backgroun d charges are thus avoided. By comparing the CCE values for different polymers, we can learn much about the effect of polymer structure on electroch romic properties. For example, with a homologous series of poly(3,4-alkylenedioxyt hiophene) (PXDOT) derivatives, we were able to show that increasing the steric bul k of the alkylenedioxy ri ng results in larger CCE values.27 This can be attributed to a more open polymer film morphology induced by the more sterically demanding rings, wh ich allows higher doping levels and thus higher contrast ratios, through suppressi on of the visible absorbance bands. Rauh et al. measured coloration efficiency () as a function of doping level by injecting a certain amount of charge into a polymer layer galvanostatically.28 The value is initially linear with inj ected charge and reaches a maximum. At higher doping levels, due to the saturation of %T values, values drop substantially, suggesting that chargeconsuming side reactions take place. We have also used this method for our absorptive/transmissive type devices based on solution processed EC polymers and have observed a similar trend during th e coloration/bleaching process.25 Another method of EC polymer character ization commonly used is the use of single-wavelength spectrophotometry to monitor switching speeds and c ontrast ratios at max. The experiment is performed usi ng the same experimental setup as spectroelectrochemistry and serves as an info rmative complement. Here a film is stepped from a potential in which the polymer is neut ral to a potential in which the polymer is fully doped. The percentage transmittance at the max of the neutral polymer is monitored


12 as a function of time as the polymer is re peatedly switched. This experiment gives a quantitative measure of the speed with which a film is able to switch between states. As with CCE, it is found that polymer structures that favor a more open morphology give rise to higher contrast ratios and faster switching speeds. Multi-Color Electrochromic Polymers–Color Control In the field of EC material s, one of the great strengths of conjugated polymers is the ability to tailor the EC properties via modification of the polym er structure. Through band gap control, one can vary the accessible color states in both the doped and neutral forms of the polymer. Numerous synthetic st rategies exist for t uning the band gap of conjugated polymers.32 In practice, this band gap control is achieved primarily through main chain and pendant group structural modification. In the simplest approach, substitution of the parent hete rocycle is used to control the band gap through induced steric or electronic effects. Homopolymerization of comono mers or copolymerization of distinct monomers also gives rise to a modification of main chain polymer structure and allows for an interesting combination of the properties supplied by each monomer unit. Additionally, conjugated polymers can be utilized in blends,33 laminates,34 or composites35 to affect the ultimate color exhibite d by the material, however here we shall only consider color control which derives di rectly from modification of the chemical structure of a conjugated polym er. Using PEDOT as a platform , several approaches have been used to produce a wide variety of multi-color, variable gap electrochromic polymers. Two such methods, chemical modification of the monomer and copolymerization, have proven to be effec tive routes. Using PEDOT as the basis for multi-color EC polymers, below we discuss a few representative examples from the


13 literature to illustrate other concepts of colo r control in conjugated polymers. This brief overview is not intended as an exhaustive review. While soluble, processable EC polymers are starting to develop into potentially useful materials,25, 36-38 electropolymerization has long been the mainstay of EC polymers. It is this route which has generate d the greatest variety of structurally diverse EC polymers. Figure 1-3 shows fifteen pol ymers as examples of how structural modification of the monomer repeat unit is us ed to tune the band gap and achieve multicolor electrochromic polymers through hom opolymerization. Color swatches based on CIE 1931 color coordinates are gi ven where available. PANI ( 1 ) has multiple colored forms depending on the oxidation state of the polymer film which includes leucoemeraldine (bright yellow), emeraldine (green), and pernigraniline (dark blue).16, 39, 40 Poly(N-methyl pyrrole) (PN-MePy) and poly(3-methyl thiophene) (P3MeTh) ( 2 3) have shown stable and reversible electrochr omism which later encouraged researchers to develop derivatized pyrrole a nd thiophene based polymers with improved electrochromic properties. Polymers 4 10 were developed to demonstrate the breadth of colors available in doped and neutral forms with relatively minimal change in structures. PProDOT-(Me2) ( 4 ) (poly(3,3-dimethyl-3,4-dihydr o-2H-thieno[3,4-b]dioxepine)) (Eg = 1.7eV) is a representative PXDOT derivative that shows little difference in color relative to PEDOT (Eg =1.6eV) as they are both cathodically colori ng; deeply colored in their neutral states and highly transmissive upon oxidation.30


14 S O O n N O O H n N H n O O N n O O SO3 Na+N S S O O O OnN S S O O O OnN S S O O O OnN N S S O O O OnO O R1R2S S n S S Si S Sn1 2 34 567 8910 0 +0 +0 I +0 + 0 I +0 +-0 + 0 +S n N H n S N OO R n N S N NnN n11 12131415 Figure 1-3. Representative electrochromic pol ymers. Color swatches are representations of thin films based on measured CIE 1931 Yxy color coordinates. Key: 0 = neutral; I = Intermediate; + = oxidized; – and – – = reduced. PEDOP ( 5 ) [poly(3,4-ethylenedioxypy rrole)] is a representa tive PXDOP (poly(3,4alkylenedioxypyrro le)) derivative.41, 42 Here the electron rich pyrrole gives rise to a material exhibiting a band gap of 2.0 eV and thus a red neutral st ate and transmissive blue oxidized state. PProDOP ( 6 ) illustrates how a slight structural modification of the monomer structure relative to PEDOP can re sult in a drastic change in the accessible color states. Here, PProDOP with a band gap of 2.2 eV, exhibits an orange neutral state,


15 an intermediate brown state and a gray/blue ox idized state. Further modifying the repeat unit through N -substitution results in N PrS PProDOP ( 7 ) [poly( N -sulfonatopropoxy ProDOP)]. Here the effect of Nsubstitution is to drastically increase the band gap to a value of 3.0 eV as a result of steric interactio ns between polymer repeat units based on the bulky sulfonatopropoxy substitu ent. As a result, this polymer is anodically coloring, changing from a completely transmissive and co lorless neutral state to an absorbing light grey oxidized state.43 PBEDOTN MeCz ( 8 ) [poly(bis-EDOTN -methylcarbazole)]44 is a three-color electrochromic polymer formed from a multi-ring monomer (comonomer). Here the neutral polymer is a higher gap material (Eg = 2.5 eV), as the 3,6linked incorporation of the carbazole into the main chain limits th e extent of conjugati on. Upon oxidative doping, this polymer shows two distin ct redox processes and thus two additional color states, green at intermediate potentials (radical cation) and blue wh en fully oxidized (dication). PBEDOT-Pyr ( 9 ) [poly(bis-EDOT-pyridin e)] and PBEDOT-PyrPyr ( 10 ) [poly(bisEDOT-pyridopyrazine)] are also examples of multi-ring monomers or comonomers that exhibit multi-color electrochromism.45, 46 Here, the donor-acceptor e ffect yields materials with low band gaps, which are capable of undergoing both pand n-type doping. For PBEDOT-Pyr the band gap is 1.9 eV due to the relatively weak pyridine acceptor. The polymer shows three distinct redox states (n -doped, neutral, and pdoped) and thus three colors. For PBEDOT-PyrPyr, the pyridopyrazin e unit serves as a better acceptor than pyridine and the result is a si gnificantly lower band gap polymer (1.2 eV) and a fourcolor state material with two n-doped states : a neutral state, a nd a p-doped state.


16 Polymers 11-15 in Figure 2 are representative exam ples of other EC polymers from the literature. Poly(benzo[ c ]thiophene -N -2-ethylhexy-4,5-dicarboxylic imide) (EHIPITN) ( 11 ) and its alternating copolymer w ith PEDOT are low band gap, n-type polymers which proved useful for their EC changes beyond the visible range in the NIR region.38, 47, 48 Polymer 12 (PPTZPQ) [poly(2,2Â’-[10-met hyl-3,7-phenothiazylene]-6,6Â’bis[4-phenylquinoline])] was described by Fungo et al.49 which turns from yellow to red upon oxidation. Polymer 13 {(PBEDOT-B(OR)2) [poly(bis-EDOT-dialkoxybenzene)]}50-52 is an example of a Bis-EDOT-arylene polymer. Th is class of polymers was pioneered by the ReynoldsÂ’ group and utilized by several other groups.50, 53 In the case when a dialkoxybenzene is used as the arylene un it, the polymers exhibit low oxidation potentials, good stability to multiple switches, and two distinctly colored states. For the case of unsymmetrically substituted polymer 13 (R1 = 2-ethylhexyl, R2 = CH3, Eg = 1.95 eV), a deep blue-purple neut ral state is observed along with a nearly transparent light blue oxidized state. For the case of th e symmetrical dialkoxybenzene analogues (R1 = R2 = heptyloxy or dodecyloxy),54 the band gap is found to be 1.95-2.0 eV as well, but in these cases the polymers are pale red in the neutral state and deep blue in the oxidized state with a green color stat e observed at intermediate potentials. When the polymer is fully oxidized, it bears a transmissive blue state. This serves as furt her proof that slight variation of repeat unit struct ure can drastically affect the colors exhibited by a polymer. Polymers 14 and 15 represent two alternative appro aches to the synthesis of EC polymers. Polymer 14 [poly(thieno[3,4b ]thiophene)] utilizes the polymerization of fused ring monomers in order to achieve an esp ecially low band gap (0.85 eV) electrochromic


17 polymer.55, 56 As a final example, polymer 15 illustrates an effective method of band gap control through the use of s ynthetically defined discre te conjugation length EC polymers.57 Here, the incorporation of a silicon linker between two bithiophenes limits the polymer conjugation length to four thi ophene rings. As a result of this simple chemical modification, the polymer changes from a bright yellow neutral form to a dark green oxidized form as opposed to the normal red to blue electrochromism exhibited by polythiophene. Thus, as the previous examples have illustrated, by varying the chemical and electronic nature of the monomer, one can vary the color of the polymer and induce multi-color electrochromism. Polymer Electrochromic Devices An electrochromic device (ECD) can be e nvisioned as an electrochemical cell where optical changes occur upon electrochemical reactions of two or more redox active materials separated by an ionic conducting layer. Electrochromi c switching of these devices is limited by diffusion of ions fr om one layer to another. ECDs based on inorganic electrochromes generally exhi bit slow switching rates (multi seconds) compared to liquid-crystal displays (L CDs), where optical changes occur through alignment of molecules under an applied electric field. However, LCDs depend on the viewing angle, are costly to process, and multiple colors cannot be obtained without addition of dyes.13 Efforts into making faster, more st able, and higher contrast ECDs have resulted in a remarkable increase in the numbe r of patents and research papers, especially after the introduction of conjugated polymers as electrochromic materials. By judicious selection of electrochromic materials and by novel ECD designs, electrochromic switching rates of 1-10 Hz can be obtained. Th e long term stability issues, often a major drawback for ECDs based on polymers, have now been overcome by introduction of air


18 stable polymers, novel polymer systems (blends, copolymers, composites, laminates, etc.), and new ionic medi a such as ionic liquids. 58, 59 Coloration efficiency values of 5003,000 cm2/C can be attained due to the low charge requirements of the conjugated polymers. The availability of many solu tion processible polymers has eased the fabrication of large area ECDs. Adapting th e currently available patterning methods, micro-structured ECDs have emerged. Here we review some of the most recent ECD systems based on the conjugated polymers a nd counterparts. More information on ECDs from metal oxides and small organi c molecules can be found elsewhere.60-62 Absorption/Transmission ECDs An absorption/transmission type ECD opera tes by reversible switching of an EC material between a colored (absorptive) and a transmissive (bleached) state on a transparent, conducting substrate. To achieve high contrast va lues in such a device, two complementary polymers are used, namely a cathodically coloring polymer and an anodically coloring polymer, deposited onto transparent electrode s (e.g. ITO on Glass, ITO on PET, or PEDOT/PSS on PET), and separa ted by an electrolyte (viscous gel or solid) to allow ion transport . The anodically coloring polymer is usually a high band gap polymer and appears transmissive in the neutral state. Upon oxida tion, it colors absorbing light in the visible region. The cathodically coloring pol ymer has a low band gap and is colored in its neutral (undoped) state becoming transmissive upon oxidation. Therefore, when both polymers are sandwiched together and an external voltage is applie d, the device switches between a colored state and a transmissive state. Th is type of device design has found use for applications such as smart windows and optical shutters.


19 Conjugated polymers have been used in several types of ECD systems as anodically and/or cathodical ly coloring materials. Poly aniline (PANI) was commonly used as a complementary electrode with me tal oxide electrochromic layers such as tungsten oxide (WO3). 63-66 Leventis et al. used surface confined composites of polypyrrole-prussian blue (anodically coloring) with a polyviologen as the cathodically coloring material.67 Other examples include a dodecyl sulfate derivatized PPy coupled with WO3 to obtain %T of ~45% at 600 nm68 and a charge balanced device of WO3 using poly(3,4-ethylenedioxyt hiophene-didodecyloxybenzene) (PEB) as the cathodically coloring polymer.28 Most recently, Tung and Ho used PEDOT/Prussian blue couple to fabricate ECDs with coloration efficiency values of ~300 cm2/C.69 ECDs with all polymer electrochromes have been widely studied in the literature. Using ITO coated plastic substrates, many co mplementary colored polymers have been investigated to obtain flexible and polymer based ECDs.70-74 DeLongChamp and Hammond have used the layer-by-layer assemb ly method to deposit soluble EC polymers electrostatically on ITO electr odes and have fabricated co mplementary ECDs by pairing PEDOT and PANI.75 The layer-by-layer electrostatic adsorp tion of a sulfonated derivative of PEDOT has been investigated by our group where the multi-layer thin films exhibit a fast and reversible redox switching behavior in aqueous media.76 Our group has optimized the visible region ab sorption of two polymers so that they could give an optimized contrast ratio in a window type ECD when they operate in a complementary fashion.77 PProDOT-(Me)2 is used as the cathodically coloring polymer due to its outstanding contra st in the visible region ( %T = 78% at 580 nm). A high band gap, pyrrole based polymer N PrS PProDOP (Eg =3.0 eV) was used as the


20 complementary anodically coloring polymer. The device possesses a %T of 68% at 580 nm ( max for the device) and switches between stat es in ~0.5 seconds under a bias voltage of 1.5 V. In this way, high contrast ECDs based on conjugated polymers can be reproducibly constructed. As discussed earlier, colorimetric analysis is a useful method for investigating the electrochromic properties of ECDs, providing informa tion on color and relative luminance. In addition, this is a valuable method for measuring the stability of ECDs upon repeated redox switching. Specifically, th e initial change in relative luminance ( %Y) of the PProDOT-(Me2)/N PrS PProDOP device is 55%. Th e long-term stability of this luminance change was monitored over th e course of several days during repeated switching between states. Initially a 10% loss in contrast was observed during the first 500 switches. However, after this conditioning period, contrast degradation slowed, with the ECD losing only 4% of its contrast afte r an additional 20,000 sw itches demonstrating the potential for high stability of conj ugated polymer electrochromic devices. Two examples of truly all polymer ECDs have been recently reported where the ITO layer has been replaced by highly c onducting PEDOT/PSS to achieve all-polymer ECDs. PEDOT/PSS films are processed fr om an aqueous dispersion which is commercially produced in large quantities by Bayer A.G. (Baytron-P) and Agfa-Gevaert. Researchers from Linköping University and Ac reo have combined an electrochemical transistor with an ECD to build an active matrix paper display.78 We have constructed ECDs using different complementary pair s of EC polymers on PEDOT/PSS coated transparent plastic electrodes and have demonstrated that PEDOT/PSS is an excellent replacement for ITO (See Chapter 5 for de tailed discussion of these devices).79


21 Reflective ECDs Electrochromism is not limited to visible color changes, but can be extended to encompass materials that exhibit radiation mo dulation in the near infrared, mid infrared and microwave regions.80, 81 This has provided the impetus for developing ECDs that can operate at longer wavelengths, beyond the vi sible region, with long lifetimes and fast redox switching times. Bessiere et al. have r ecently reported an IR modulator ECD using powder hydrates of tungsten oxide embedded in a plastic matrix with contrast values of 30-50%.82, 83 Other IR modulating devices based on WO3 include studies by Hale and Woollam84 and Franke et al.85 Polymer based devices comprising PANI-CSA as the active EC material have been used for therma l emissivity control in the NIR and mid-IR region (2.5-20 m).86-89 PEDOTÂ’s IR electrochromism has been studied by Pages et al. in broadband ECDs using porous gold electrodes where they optimized the pore size and gold thickness for reflectance analysis.24 In order to characterize the infrared EC properties of the polymers synthesized in our labs, we have employed a flexible, out ward facing, reflective device platform originally developed by Bennett and Chandrasekhar.90, 91 A device was constructed by electrosynthesizing PProDOT-(Me)2 as the surface active EC polymer (due to its outstanding contrast ratio and high stability) onto a slitted (slit separation ~1-2 mm) goldcoated Mylar reflectiv e conducting substrate.92 As this film is switched from its neutral, colored state to its oxidized, bleached state, a color change of the ECD from absorptive blue to reflective gold take s place in 3 seconds. In th e visible region, EC switching yielded a reflectance contrast ratio of 55% at 600 nm. In the NIR regi on, the contrast ratio was as high as 90% at 1.8 m.


22 ECD Applications The most common applications of EC materi als include a variety of displays, smart windows, optical shutters, and mirror devices. Below is a list of leading companies that do research in electrochromics field and their state-of-the-art technology on electrochromics. A significant amount of in formation about electrochromism and related applications can be found on the world wide web.93 Dow Chemical. The Dow Chemical Company is de veloping a low-cost electronic display based on printed elec trochromic inks. COMMOTIONTM technology has been specifically developed for use in novelty and promotional products, including Radio Frequency Identification (RFID), smart label, and packaging applications. COMMOTION has already been used by a UK retailer, Marks & Spencer, for an animated greeting card application. Marks & Spencer sold the card for £3 each. Other applications include smart packaging, which when combined with RF ID smart labels will provide shelf-edge marketing. This technology was first reported in a forum organized by the Technical Association of Graphic Arts in October, 2002. A family of screen printable electro-active inks (materials not specified) is developed by Dow for Reflective ElectroActive Displays (READ). Sage Electrochromics. Sage Electrochromics was founded in 1989 and has produced an electronical ly tintable window called SageGlass which is intended to be used as power saving windows for buildings, sunroof s, protective eyewear, etc. The window is consisted of a sputtered inorganic thin film electrochromic layer (possibly a metal oxide film although not disclosed in the website ) and a ceramic ionic conductor sandwiched between two transparent conductors. SageGlass is claimed to block 95% of the sunlight. Prototype devices showed lifetimes of ove r 100,000 switching cycles with no noticeable


23 degradation. The cost and switching speed are not clear. However, the following statement: “SageGlass® windows tint and clear gradually an d uniformly. This is a good thing, since too-fast switchi ng of the glass can thermally shock it causing stress and possible shattering” suggests that they do not switch fast. Gentex . Gentex was founded in 1974 and is best known for their electrochromic, automatic-dimming mirror called NVS (Night Vision Safety). Auto-dimming mirrors detect glare and driver’s vi sion is protected by automatical dimming. Their mirrors are offered as standard or optional equipment on over 200 vehicle models. Gentex uses solution-phase electrochromic 4,4’-bipyrid ines (viologens). They sandwich an electrochromic gel (viologens dissolved in a viscous electrolyte) between two pieces of glass, each of which has been treated with a transparent, electric ally conductive coating, and one with a reflector. The switching speed depends on the diffusi on of viologens in the gel and is typically in the order of seconds. When a potential is applied, mobile viologen molecules will diffu se to both electrodes whic h results coloring. Once the potential has been removed, the charged species mix, transfer their charges, and the color dissipates from the system. Power must be ap plied continuously to maintain coloration (no open circuit memory). Donnelly. The Magna Donnelly SPM™ EC Mirror is similar to that of Gentex’s in terms of the electrochromic device design a nd materials. The only difference is that Donnelly uses an SPM™ (Solid Polymer Matrix) technology which replaces the gel electrolyte with a solid polymeric conductor. So there is no leaking ev en if the glass is cracked. In the market for auto-dimming el ectrochromic mirrors, Donnelly has 16% of the market, compared with Gentex's 80%.


24 DynamIR Corp. DynamIR (founded in 2002) is an a ffiliate of Ashwin-Ushas Corp (founded in 1992) which develops visible and IR electrochromics technology. They focus on IR camouflage reflective devices, electrochr omic sunglasses, and spacecraft thermal controllers using conducting polymers as the electrochromic material (mainly PANI). Their devices are light-weight (~0.12 g/cm2) and thin (~0.5 mm) with switching speeds of ~ 2 seconds. NTERA. NTERA is a Dublin-based company founded in 1997 which develops nanomaterial-based product appl ications. NTERAÂ’s NanoChromicsTM technology allows for fabrication of display devices benefiti ng from high surface area of nanostructured semiconducting metal oxides chemically bound to electrochromic viologen molecules. Devices comprise a reflector made of a nanos tructured film of Titanium Dioxide (the same chemical used to make paper white) which provides a solid and highly reflective white background. The colored viologens in front of this reflective background have the appearance of ink. The front transparent electrode is micropatterned for display applications. The switching speed is in th e range from milliseconds to seconds. Their likely applications include public informa tion signs, point-of-sal e signs, and e-books. A U.K. based company Densitron will ma nufacture their display units. Pilkington. Pilkington introduced its first commercial electrochromic smart window product on glass in late 1998. Pilkington is currently the only manufacturer able to produce large-area devices at an acceptabl e quality level and its windows are being tested by Lawrence Berkeley Laboratorie s for office windows. Called Pilkington EControl, electrochromic windows comprise a tungsten-bearing electr ochromic layer and changes color from clear to blue on demand.


25 Cidetec. Cidetec of North Spain has introduced polymer based electrochromic false finger nails, probably the oddest application the fi eld has encountered. The electrochromic nail is made up of a number of superimposed laye rs. Sandwiched between these are transparent conducting oxides and a number of electrochromic polymers as well as an electrolyte for ionic interchange be tween the electrochromic polymer layers. A digital control device is used to program th e desired color on the nails. They are also developing smart windows and a Catalan compa ny, Cristales Curvados S.A. will launch their first windows to the market in year 2004. To a lesser commercial extent, compan ies including Avery-Dennison and Saint Gobain have EC programs. In additi on, researchers from Lawrence Berkeley Laboratories have installed and tested smart windows for office rooms in Oakland, Ca. and the National Renewable Energy Labor atories (NREL) has ongoing research on developing prototypes of ver tically integrated, photovolta ic powered electrochromic displays. In a different application, the optic al change of chromoge nic materials due to proton intercalation is promising for hydr ogen sensor applications and has been demonstrated by NREL researchers using WO3 as a molecular hydrogen sensor. General Patterning Methods Patterning of electrodes for electronic device s is essential for fabrication of finestructured electronic circuits, independen tly addressed displa y devices with high resolution values and for devices which requir e separation of adjacent electrodes. In this section, general patterning methods to make structured electrodes will be discussed in detail. These methods include conventional lithographic techniques, soft lithography, and other printing techniques. Ta ble 1-1 lists the most comm only used patterning methods along with their resolution limits. To date, onl y few of these methods have been utilized


26 for electrochromic devices by our group and others. These will be explained separately in the section entitled “Patterning of ECDs.” Optical Lithography Optical lithographic techniques involve e xposing an irradiation sensitive polymer resist layer to a high intensity deep UV li ght (157 nm to 248 nm) through a mask and changing the chemical structure of the resist which results in a change in solubility.94 The next step is to remove the exposed resist by solvent etching or plasma etching. The resulting pattern can then be used to sele ctively deposit on the substrate or to introduce dopants. Optical lithography has been a standard large-scale fabrication process used by the semiconductor industry. Its resolution is lim ited by diffraction of the incident light according to the Rayleigh equation: R = k /NA where k is an empirical constant depending on the photoresist or the mask used, is the wavelength of the laser light, and NA is the numerical aperture of the optical syst em. In practical, it is reasonable to assume that the resolution is on the order of the wavelength of the light used. Several other optical lithography techniques have been develo ped to give better re solution values such as extreme UV (13 nm)95 and soft x-ray (~2 nm) lithog raphy, but these methods suffer from high cost and lack of suitable photoresists. Electron Beam (e-beam) Lithography Electron beam (e-beam) lithography involve s bombardment of a substrate with high energy electrons (~10 -200 keV) with resolution values of a few nanometers depending on the beam size. Contrary to optical lithography, the e-beam method can directly write on a substrate from a comput er designed pattern. During the process, the electrons slow down and this results in a dept h gradient, hence the loss of depth of focus.


27 Limited writing speed and high operation cost prevented this method from use in mass production. It is mostly being used to pattern masks for op tical lithography. Table 1-1. Patterning methods, the highest resolution values achieved from these methods, and their brief description. Patterning Method Highest Resolution Brief Description Conventional Lithography Optical Lithography ~ 150 nm; limited by light diffraction Photon dependent direct writing techniques (e.g laser ablation). Complicated and expensive Electron-beam Lithography Few nanometers, limited by scattering of electrons Direct writing from a computer designed pattern using a high energy electron source Scanning Probe Lithography ~100 nm SPM tip (ultra-micro electrode) writes lines of polymers on the substrate. Soft Lithography Microcontact printing ( CP) ~30 nm Patterning of m onolayer using a stamp allowing area selected deposition. Printing Techniques Inkjet printing ~50 m Commercial inkjet printers are modified to print soluble materials. Resolution is limited by substrate wetting and printer. Screen Printing ~20 m Requires processible (soluble) materials Line Patterning 5-30 m Uses the difference in reaction with the substrate and the printed lines on it. Scanning Probe Lithography This method involves nanometer-scale dir ect writing on a substrate using a sharp probe of a scanning probe microscope (SPM ). SPM was first discovered by Binnig and Rohrer in 198296 which led to a Nobel Prize in Physics in 1986. An SPM probe can pattern a material by manipulating molecules in close proximity to the sample through a tunneling current (conducting s ubstrates) or vertical m ovement of the probe (nonconducting substrates). Atomic force microscopy (AFM),97 commonly used to image


28 non-conducting substrates with nanometer reso lution, can be used to pattern surfaces through anodic oxidation or selected etching of selected regions.98 One example of this type of patterning is Dip-Pen Nanolithogr aphy which was described by Maynor et al.99 They printed nanowires of PEDOT on insu lating surfaces via elec tric-field induced polymerization of EDOT at the AFM tip. By applying a pot ential between the monomer coated AFM tip and the surface, well-def ined PEDOT lines were generated with dimensions less than 100 nm. Microcontact Printing ( CP) Microcontact printing ( CP) is based on the transfer of an organothiol ink to a substrate (usually gold) using an elasto meric polydimethylsiloxane (PDMS) stamp.100-102 Stamps are initially prepared by casting a nd curing of PDMS on a “negative” master pattern and they can be used repeatedl y. Self-assembled monolayers (SAMs) of organothiols (1-3 nm thick depending on the le ngth of alkyl chain) selectively cover the surface by contact which is then used as a ma sk for etching. Alternatively, area selected electropolymerization can be performed on th e exposed (uncovered) regions followed by removal of the SAM mask. It is a simple and inexpensive non-li thographic technique which yields pattern dimensions down to ~30 nm in size. In cont rary to lithographic techniques, it does not require clean rooms, which makes th is technique accessible to chemists and material scientists. It has alr eady proved useful for fabrication of organic electronic devices such as transistors, polymer light-emitting diodes, and electronic paper.103, 104 Despite the lack of an example in the cu rrent literature, this method is highly suitable for patterning of electrodes for pol ymer electrochromic devices since an


29 electrochromic conjugated polymer can be selectively electro-deposited on a SAM modified electrode with high re solution. The only application of CP in electrochromism was recently reported by Admassie and Inganas.105 They have patterned an electrochromic layer of spin coated PE DOT/PSS on ITO by putting a rubber stamp on top of the wet polymer, drying, and removing the stamp. Upon removal, fine gratings (600 lines/mm) are left behind on the PEDOT/PSS la yer to yield alterna ting lines of polymer on ITO. They have then compared the elec trochromic properties of the patterned and unpatterned films to show that the patterned films give significantly higher absorption values and higher contrast ratios between th e colored and transmissive states when switched. They have attributed this to loss of film transmission due to the diffraction of the incident light by the grating. Inkjet Printing Inkjet printing relies on modi fication of a commercial inkjet printer to transfer droplets of a soluble material onto a substrate to form a desired pattern.106 It has been extensively used for printing soluble conj ugated polymers to fabricate multi-color polymer light-emitting diodes (PLEDs) and is considered to be one of the likely technologies to be used in the manufacture of PLEDs.107 Polymers printed this way are restricted to low viscosity, th erefore low molecular weight. In order to obtain uniform deposition, drop on demand inkj et printing (bubble-jet) is used which yields high placement accuracies. Resolution of inkjet printing is somewhat lower (~50 m) compared to other techniques due to the latera l displacement of the printed ink before it can wet the substrate.


30 Patterning of ECDs In order for EC polymer display technol ogy to evolve towards higher definition devices, new methods for active material de position must be developed. Therefore, a significant amount of attention has been directed towards information displays that require a high degree of visible color cont rast. Typical device construction is based on sandwich-type configurations, si milar to the ones discussed earl ier, where at least one of the electrodes is transparent. An emer ging facet of ECD c onstruction pursued by researchers is the metallization of a surface via patterning methods. This is useful since it allows the combination of at least two polymers at both large (centimeter) and small (micron) scales that can display a set of co lors on a surface, or be averaged by human visual perception by color mixing. Moreover, metallization to form contact electrodes may be performed on ionic-permeable material s in order to develop reflective/absorptive surfaces with especially rapid switching rate s. For example, Chandrasekhar et al. have used porous electrodes to inve stigate PANI based flexible devices for spacecraft thermal control applications with contrast va lues of 40-50% in the mid-IR region.81 We have recently utilized metal-vapor deposition and the line patterning process developed by Hohnholz and MacDiarmid108 to deposit conjugated EC pol ymers for the construction of novel ECDs. This section presents a brief revi ew of some of the patterning techniques that are used to fabricate electrodes fo r polymer ECDs. Many other patterning and printing techniques might be applied in the future depending on the needs for resolution, cost, and accessibility. Metal-Vapor Deposition ECDs have been constructed employi ng porous polycarbonate membranes that have been metallized with a thin layer of gold. Specific gold patterns have been deposited


31 by attaching a physical mask to the naked subs trate prior to metal deposition. Typically, a 50 nm layer of gold is sufficient to yiel d a well-adhered shiny gold electrode, while maintaining the porous nature of the flexible electrode. This point is important given that high surface reflectivity is required to afford a useful vi sible/NIR contrast, and porosity is necessary to facilitate ion flux in the final device (See the porous type device scheme in Chapter 2). Conjugated polymers are electrosynthesized directly onto the gold surface or can be sprayed and solution coated.25 Using the electrodeposition method, multiple polymers can be incorporated into the same array-type device by first depositing one polymer, and after washing with monomer-fr ee electrolyte solution, the second polymer is electrodeposited onto the array. Porous-t ype patterned ECDs c onstructed with PXDOT polymers as the active layer as are discussed in detail in Chapters 3 and 6. The simple concept of color and contrast in these primitive displays evokes conceptual thinking of higher resolution pixel devices and provides the basis for the construction of lateral ECDs on flexible substrates. Line Patterning Line patterning (first repo rted by Hohnholz and MacDiarmid108) is an excellent method to build fine structured electrode s on surfaces such as plastic or paper. Metallized electrodes in the sub-millimeter range have been prepared by initially printing a black ink pattern “negative” onto a flexible substrate. The substrate, together with the ink pattern, is then metallized with gold109 via an electroless deposition method. Following metallization, the ink “negative” is remove d by sonication in toluene to produce a patterned electrode. Laterally configured dual polymer ECDs that have been constructed utilizing this method are discussed in Chapter 4 of this dissertation.


32 Screen Printing Screen printing is an additive patterning method where the desired material is selectively deposited through a template ma sk with resolution values of ~20-100 m. Introduced for electroactive polymers by Garnier et al. for printing el ectronic circuitry of polymer FETs,110 there are only a few examples of th is technique for ECD applications. Coleman et al.111 used this method to pr int electrical contacts fo r finely patterned ECDs. Brotherston et al.112 have demonstrated checkerboar d and stripe patterned ECDs comprising color mixing PEDOT and V2O5 as electrochromic materials. Andersson et al.78 of Acreo have combined an organic tr ansistor with a display ECD all based on organic materials using screen printing. In this work, solution processable PEDOT has been printed on a paper both as the transistor component and the ac tive EC material to produce smart pixels. Structure of Dissertation The main characteristics of this work are the optical and electrochemical characterization of conjugated polymers in di fferent electrochromic device platforms and applications of patterning met hods to generate structured el ectrodes to be used for these devices. Chapter 2 mainly summarizes the experimental methods which are used to pattern electrode materials a nd elucidates the characteriza tion methods that are mainly used for electrochromic polymers and devices. Surface-active reflective ECDs and their pa tterning are investigated in Chapter 3 for their fast switching capabilities, power c onsumptions, and highly efficient operations. A new method to make contacts for porous el ectrodes is described. This method allows hiding unattractive contact lines for displa y type devices without compromising the


33 device operation. An example of a pixelated num erical display device is also shown to demonstrate the use of patterning to create highly contrasted surfaces. Chapter 4 describes the use of a new pa tterning method, namely “Line Patterning,” to make lateral electrochromic devices. It is a simple way to pattern polymer and metal electrodes since it does not require complicated lithogra phy and results in decent resolution values for micro-structured elec tronics. As an example, interdigitated gold electrodes are generated to esta blish the variation of the el ectrochromic switching time as a function of the distance between the interd igitated lines. Chapter 5 introduces highly conducting PEDOT/PSS films as electrode mate rials for transmissive type ECDs which can replace the conventional ITO electrodes. It further evaluates the line patterned PEDOT/PSS electrodes for electrochromic devi ce applications. A truly all polymer ECD is fabricated comprising PEDOT/PSS electrode s, electrochromic polymer layers, and a polymer based gel electrolyte. Solution processability of conjugated polymer s is an important factor for accurate polymer characterization and large area appli cations. Chapter 6 deta ils the optoelectronic characterization organic so luble PXDOT derivatives which are spray coated onto conducting substrates. Spray coated poly mer thin films are compared with electropolymerized films for their performan ce both in electrolyte solutions and ECDs.


34 CHAPTER 2 EXPERIMENTAL METHODS This chapter provides background informa tion on experimental methods employed for patterning of electrodes, electrochemical and optical characterization of conjugated polymers, and fabrication of electrochromic devices. These methods will be frequently referred to throughout the subsequent chapters. Chemicals and Materials Reagent grade propylene carbonate (PC) and acetonitrile (ACN) in Sure Seal were purchased from Aldric h. ACN was distilled over CaH2 before use. Tetrabutylammonium hexafluorophosphate (TBAPF6), tetrabutylammonium perchlorate (TBAP), lithium perchlorate (LiClO4), and poly(methyl me tacrylate) (PMMA) (Mw ~ 350,000 g/mol) were purchased from Aldrich an d used without any further purification. EDOT and PEDOT-HAPSS were obt ained from Agfa Gaevert . ProDOT,29 its dimethyl derivative ProDOT-(Me2),30 BEDOT-NMeCz,113 and BEDOT-B(OC12H25)2 51 were synthesized as reported previously. PProDOT-(CH2OC18H37)2, and PProDOT(CH2OEtHx)2, were prepared according to me thodologies previously reported.37 ITO coated glass slides were purchased from Delta Technologies (50 x 7 mm and 75 x 25 mm). Prior to use, the slides were soni cated in distilled water, then acetone for 15 minutes to remove any inorganic and orga nic residue followed by air drying. Tracketched polycarbonate membranes (200x250 cm sheets) were purchased from GE Osmonics Inc. Membranes are 10 m thick with 10 m diameter cylindrical pores. Nominal pore density of the membranes is 105 pores/cm2 as reported by the manufacturer.


35 99.99% pure gold coins were purchased from a lo cal coin store (National Coin Investors Inc) and cut into 1cm x 2cm pieces for me tal vapor deposition. Nickel slugs were purchased from Aldrich. Gold coated Kapton substrates (100 nm gold on 1 mm Kapton) used for counter electrodes of reflective type ECDs were purchased from Astral Technology. Adhesive conductors (1/4” wide copper tape) to make electrical contacts to the ITO, gold and nickel electrodes were purchased from 3M. Preparation of Electrodes Metal Vapor Deposition Gold and nickel deposition on porous me mbranes was carried out using a high vacuum thermal evaporator (Denton DV-502A ). Figure 2-1 schematically shows the deposition of gold onto a masked membrane . During the metallization process, the membrane (5 x 5 cm) was sandwiched between a clean piece of glass and an aluminum shutter mask (prepared in the UF Chemistr y machine shop) to pattern the membrane surface. Gold was placed on a tungsten bol t and heated by applying ~150 Amperes between the two ends of the boat. The metallization was carried out at 10-6 to 10-5 Torr at with a deposition rate of 4.0 Angstrom/s to yi eld shiny metal surfaces with a thickness of 50 nm, as measured by a Sloan DEKTAK 3030 pr ofilometer. The temperature inside the chamber can be as high as 150 200 C which may yield shrinki ng of the polycarbonate membranes (the Tg of polycarbonat e is around 150 C) and this may cause permanent wrinkles on resulting electrodes. For exam ple, the process completely destroys polypropylene membranes which degrade at temperatures above 150 C. It is important to fix the membranes tightly between the s upport and mask to minimize the wrinkling.


36 Gold deposition on fiber-like membranes su ch as laboratory filter paper requires thicker gold layers (~150 nm) to obtain low surface resistance values. Gold on these membranes are not as reflective as on the polycarbonate membranes. Backside addressed electrodes were prepared by me tal vapor deposition of gold on both the front (active) and the back (contact) of a porous substrate through a proper mask. Support Membrane Mask HV chamber Figure 2-1. Schematic representation of hi gh vacuum metal vapor deposition process. Line Patterning First introduced by H ohnholz and MacDiarmid,108 line patterning proved useful to prepare fine structured electro des on surfaces such as plastic and paper. Line patterning benefits from the difference in physical and/ or chemical properties between a substrate and lines, which have been printed on it by a conventional copying or printing process. Figure 2-2 shows the general procedure for the line patterning process. Using computer aided design (CAD) software such as Adobe Illustrator or Microsoft Paint, a desired pattern is designed. This pattern is then inverted to yield the “negative” image and printed onto a polymer transparency film (Nashua XF-20) using a co mmercial B&W laser printer. Figure 2-2a shows an example of PEDOT/PSS patterning steps on a transparent plastic substrate. Highly conducting PEDOT/PSS solution was applied onto printed


37 substrates either by smear coating or sp ray coating. After drying under vacuum, the printed ink was removed by sonica tion in toluene for 5 minutes. (a) Plastic substrate with negative patternMetallization of non-printed regions in metallic salts Removal of printer ink in toluene Reduction of gold on metallized regions Line patterned gold electrode (b) Figure 2-2. Line patterning of plastic substrates. (a) PEDOT-PSS electrodes (b) Electroless gold deposition. Electroless Metal Plating Line patterning of nickel and gold onto printed substrates was performed using electroless metal deposition109, 114 as shown in Figure 2-2b. Th e following metal solutions were prepared to activate and deposit on non-printed areas: Tin bath: 0.01 ml of 12 M HCl was added to 100 ml of deionized water. To this solution, 10 mg of SnCl2 (0.1 g/l) was added. Palladium bath: 0.01 ml of 12 M HCl was added to 100 ml of deionized water. To this solution, 10 mg of PdCl2 was added. Plastic substrate with negative pattern Deposition of highly conducting PEDOT-PSS Removal of printer ink in toluene Line patterned PEDOT-PSS electrode


38 Nickel bath: 2.9 g of NiSO4.6H2O, 1.7 g of NaH2PO2H2O, 1.5 g sodium succinate, and 0.036 g of succinic acid were added to 100 ml of deionized water. Using a magnetic stirrer, the resulting solution was stirred fo r 10 minutes to give a green colored solution. Gold bath: This bath consists of a mixture of equal amounts of the following two solutions. The first solution consists of 0.252 g gold (I) sodium thiosulfate in 10 ml of deionized water. The second solution cons ists of 0.198 g sodium L ascorbate, 0.152 g anhydrous citric acid, and 0.112 g KOH dissolv ed in 10 ml of deionized water. Substrates were first dipped into the Tin bath for 2 minutes. This results in thin layer adsorption of Sn only onto non-p rinted regions due to the hydrophobic nature of the printer ink. These activated regions were th en exposed to the Palladium bath for 2 minutes to replace Sn with Pd. The substrates were then placed in Nickel bath at 60 C. Rapid deposition of nickel was observed. When the nickel homogenously covers all the non-printed active regions, the s ubstrates were transferred into toluene and the printer ink was removed by sonication in toluene to pr oduce the patterned elec trode. Finally, gold deposition was achieved by completely imme rsing the electrodes into a Petri dish containing the Gold bath. Gold thickne ss was controlled by deposition time. All-Polymer Electrodes 3M transparency film substrat es (PP 2500, Contact angle = 9.5o) were used without any pre-cleaning. After mixing 5% wt. diethy lene glycol or 5% N-methyl pyrrolidone with 95% wt. PEDOT-HAPSS, the solution was stirred in a flask for one hour at room temperature. This dispersion was then spin co ated onto the plastic s ubstrates at 1000 rpm. The resulting films were placed in an oven at 120 C for 5 minutes. Films were then dried in a vacuum oven overnight and stor ed in a dessicator until use.


39 Conductivity Measurements The surface resistance of the metal and al l-polymer electrodes was measured using a standard two probe method as show n in Figure 2-3a. Surface resistance, Rs, can be defined as the ratio of DC voltage, V , to the current, I , flowing betwee n two probes of a voltmeter that contact the same side of a material. Surface resi stance is a direct result of a measurement and depends on the geometry of the probes. The surface resistivity, s, is a property of the material which is independe nt of the distance and geometry of the measuring probes. It is given by the following equation115: L W R ) square / (s s where L and W are the length and the width of the measured material, respectively. (Figure 2-3a). W L Rs V I (a) (b) Figure 2-3. (a) Surface resistivity measur ement of a thin film. (b) Four-probe conductivity measurement setup. The conductivity of the PEDOT-HAPSS el ectrodes presented in Chapter 5 was obtained using four-point probe method. This method benefits from a predefined probe geometry as shown in Figure 2-3b in order to measure the conductiv ity independent of the contact area. A conductivity station incl uding a Signatone S-301-4 model four probe device, a Keithley 224 programmable current source, and a Keithley 181 voltmeter is


40 available in our labs to pe rform conductivity measuremen ts. The bulk conductivity of a sample can then be calculated from the following equation115: ) cm ( 2 ln ) Volts ( V ) Amperes ( I ) cm / S ( The four-probe method has several advantag es over the standard two probe method for measuring the bulk conductiv ity of conducting polymers. Fo r example, it eliminates errors caused by contact resistance, since th e two contact probes measuring the voltage drop (2 & 3 in Figure 2-3b) are different from the contacts applying the curre nt across the test specimen (1 & 4 in Figure 2-3b). It also allows conductivity measurement at a broad range of applied currents varying from 1 A to 100 PEDOT-HAPSS films for conductivity meas urements were prepared as freestanding films (5-20 m) and thin films deposited on non-conducting substrates (0.11 m). In the latter case, a thin film of PEDOT-HAPSS (~100 nm) was deposited by spin coating on a glass slide. Electrochromic Polymer Deposition Electrochemical Polymerization All electrochemistry was performe d using an EG&G PAR model 273A potentiostat/galvanostat. Electrochemical polymerization of th e polymer films was carried out in a 0.1M electrolyte soluti on containing 10 mM mono mer unless otherwise noted. A three-electrode cell containing a meta l-coated membrane or ITO/Glass as the working electrode, a platinum flag as the c ounter electrode, and a silver wire as the pseudo-reference electrode were used for electrodeposition of polymer films via potentiostatic or potentiodyna mic methods. The pseudo-reference silver wire was


41 calibrated vs. Fc/Fc+ by dissolving ferrocene in the el ectrolyte solution and determining the E1/2 of the Fc/Fc+ against the silver wire. Figure 2-4 shows a potentiodynamic depos ition of a dihexyl derivative of PProDOT (PProDOT-(Hx)2) on Pt button from a 10 mM PProDOT solution in 0.1M TBAPF6/ACN and represents an example for ot her polymers. At 1.05V, the electrode potential is sufficient to oxi dize the monomer to its radica l cation. Monomer oxidation is followed by coupling of radicals to form the polymer which deposits on the electrode surface. When the potential is lowered to ~ -0 .1V, the reduction of the oxidized polymer occurs. Repeated cycling of the process yiel ds more polymer deposition on the electrode which is evident from the increase of the current density between -0.3V and +0.2V (Redox couple of the polymer). -1.5-1.0- -4 -2 0 2 4 6 8 10 12 S O OJ (mA/cm2)E (V) vs. Fc/Fc+ 1.1 V 1.17 V 1.05 V Figure 2-4. Potentiodynamic deposition of PProDOT-(Hx)2 on Pt button electrode (Electrode area = 0.02 cm2). Inset shows the chemical structure of the monomer. Spray Coating The soluble polymer films were spray-coat ed onto ITO-coated glass slides (20 /sq) and gold-coated polycarbonate membranes (5 /sq) using an airbrush (Testors


42 Corp.) at 12 psi air pressure from a 0.6% w/w solution of polymer in toluene. Polymer films were then dried under vacuum a nd stored in a dessicator until use. Device Construction The composition of the gel electroly te used in the ECDs was TBAPF6/ PMMA/ PC/ ACN in a ratio of 3:7:20:70 by weight. Th e gel electrolyte was prepared by first dissolving TBAPF6 and PMMA in ACN and slowly evaporating the ACN to reach honey-viscous condition. A few dr ops of PC were added to d ecrease the vapor pressure of the gel electrolyte yielding a highl y conducting transparent gel (~3 mS/cm). Window type absorption/transmission EC Ds were constructed by pairing a cathodically coloring polymer to an anodically coloring polymer separated by a gel electrolyte. A general scheme for this type of devices is given in Figure 2-5a. One polymer was oxidatively doped while the othe r was neutral prior to device assembly. ITO/Glass and PEDOT-HAPSS/Plastic were used as the electrode material. Drying of the gel electrolyte at the edges provided sealing. A general scheme for preparing reflective type ECDs is given in Figure 2-5b. For the reflective ECDs described in Chapter 3, films of either PEDOT or PProDOT-(Me)2 was electrochemically deposited onto the count er electrode consisting of a 1.5 x 2.0 cm Au-coated plastic sheet using a deposition charge of ~150 mC from a 0.1M LiClO4/PC electrolyte. This electrode is used as an i on storage layer for the active layer and does not contribute to the optical properties of the device. The active layer of PEDOT, PProDOT or PProDOT-(Me)2 for the front working electrode was deposited on a metallized porous membrane. In order to obtain the best pe rformance of the ECDs in terms of color contrast, it is necessary to pair the activ e layer with a counter electrode containing a


43 Transparent Support Layer Transparent Electrode Anodically Coloring EC Polymer Cathodically Coloring EC Polymer Polymer Gel Electrolyte Transparent Support Layer Transparent Electrode Anodically Coloring EC Polymer Cathodically Coloring EC Polymer Polymer Gel Electrolyte (a) Polymer Counter Electrode Active polymer layer Porous substrate Gel electrolyte Porous separator Support Reflective Metal layer (b) Figure 2-5. (a) Schematic representation of an absorption/transmissive type device. (b) A reflective device scheme using porous electrodes. higher amount of electroactive polymer so th at the electrochemical properties of the counter electrode do not limit the optical contrast of th e active layer. The counter electrode was placed, face-up, onto a transparen t plastic substrate and a thin layer of gel electrolyte was homogeneously applied and the polymer coated membrane placed face up. A few drops of gel electrolyte were also added on top of the act ive layer to ensure adequate swelling of the polymer. Finally, a transmissive window was placed over the outward facing active electrode to protect the polymer film. Reflective ECDs described in Chapter 6 are constructed in the same manner except that the electrochromic polymer layer was spray coated onto gold coated membrane substrates.


44 Backside addressed pixels in the numerical display de vice described in Chapter 3 were independently switched using a Nationa l Instrument PCI-6703 analog output board. A virtual instrument (VI) program, written fo r LabView software, was used to drive the board. The program allows separate voltage control on any of the 16 output channels against a shared ground. Electrochemical Methods Cyclic Voltammetry Cyclic voltammetry (CV), is a simple a nd valuable technique for the study of electroactive polymers. The current flowing at the working electrode/solution interface is monitored as a function of th e applied potential. Both qualitative and quantitative data may be obtained and the technique finds part icular use in preliminary studies of new systems. CV shows the potentials at which oxidation and reduction processes occur, the potential range over which the solvent is stab le, and the degree of reversibility of the electrode reaction. Furthermore, repeated cyc ling reveals the electrochemical stability of electroactive species. Cyclic voltammetry of el ectroactive polymer films are often accompanied by a capacitive current which broadens the resul ting peaks due to the microporosity of the films.116 Electroactive films presented in this work can be reversibly cycled between neutral and p-doped forms in a non-aqueous el ectrolyte. Important parameters of a polymer CV are the half-wave potentials (the potential where the concentrations of the oxidized and reduced species are equal), scan rate dependence of the peak current, and the reversibility (shape) of the potential wave.


45 Chronocoulometry In a chronocoulometry experiment, the tota l charge is monitored as a function of time when a large magnitude potential st ep is applied. For a redox reaction R O + ne the redox charge passed is obtained by integrating the Cottrell equation117: t D nFAC 2 Qr r where Cr and Dr are the bulk concentration and di ffusion constant of the redox active sites, respectively. The advantage of chr onocoulometry over chronoamperometry (current vs. time) is that the integration of curr ent smoothes random noise and eliminates timeindependent current. For an absorption/transmission type ECD comprising two complementary polymers, it is important to match redox (switc hing) charges prior to device construction for balanced switching. Using chronocoulom etry, redox charges of polymer films were determined by stepping the potential from a negative extreme to a positive end, and monitoring the charge versus time in a th ree-electrode cell cont aining 0.1 M supporting electrolyte solution (e.g. TBAPF6/PC). Figure 2-6 shows a chronocoulometry experiment of a di(2-ethyl hexyl) derivatized PProDOT film on ITO when the applied potential is stepped from -0.9V to +0.6 V vs. Fc/Fc+. Th e current response of the same film during the redox switch (chronoamperometry) is also shown.


46 - -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 E (V) vs. Fc/Fc+Time (Seconds) - 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Current densityJ (mA/cm2)Time (seconds) Q (mC/cm2) Charge density (a) (b) Figure 2-6. Chronocoulometry e xperiment of a PProDOT-(EtHx)2 film on ITO. (a) The potential step (b) Current and char ge curves as a function of time. Optical Methods Reflectance Spectroscopy The reflective characterization of the elec trochromic devices was carried out using a Cary 500 Varian UV-VIS-NIR spectrophotomete r mounted to an integrating sphere. A device without the active polymer layer, but otherwise with the same construction, was used as a reference. This reference account s for the relatively low reflectance of gold at shorter wavelengths. Figure 2-7 schematically shows the integrating sphere along with possible light reflections. The sphere consists of a sample compartment and a reference compartment. Inner walls of the sphere are coated with highly reflective BaO. When the incident light (Itotal) hits the sample device, the total reflected light consists of specular reflection (angular, mirror-like) and diffuse reflection (scatte red at all angles). Specular light is reflected at an angle of 8 to the normal of the incident light and it can be trapped with a dark mirror to preferentia lly measure the diffuse light.


47 ItotalDiffusive standard BaO (specular) IR(diffuse) Is Device Ireference Figure 2-7. Integrating sphere used for refl ective characterization of surface active ECDs. Spectroelectrochemistry Spectroelectrochemistry plays a key role in examining the optical changes that occur upon doping or dedoping of an ECD. It provides information about the EC polymerÂ’s band gap and intraband states created upon doping. Measurements were carried out with a UV-Vis-NIR Varian Cary 500 spectrophotometer. ECDs were placed in the sample compartment of the instrument and connected to a potentiostat to allow potential application while monitoring th e absorption/transmission (or reflectance) spectra. An ECD without the EC polymer layer was used as the reference. Single Wavelength Transient Absorption Another method of EC polymer and device characterization commonly used is the use of single-wavelength tran sient absorption to monitor sw itching speeds and contrast ratios at max. The experiment is performed using the same experimental setup as spectroelectrochemistry and serves as an info rmative complement. Here a film is stepped from a potential in which the polymer is neut ral to a potential in which the polymer is fully doped. The percentage transmittance at the max of the neutral polymer is


48 monitored as a function of time as the polymer is repeatedly switched. This experiment gives a quantitative measure of the speed with which a film is able to switch between states. Colorimetry In-Situ Colorimetric Analysis is used as a means of precisely defining color and contrast ratios in electrochromic polymers. Th e colorimetric analysis experiment is based on a set of color coordinates, su ch as the CIE 1931 Yxy color space.18 In this color space, Y corresponds to the brightness or luminance of a color (specifically the brightness of the transmitted light in a transmission experiment ), whereas the xy coordinate of a color defines its hue and saturation. Colorimetry is based on a standard observer and thus takes into account the manner in which the human ey e perceives color. In a typical experiment, the light transmitted through a polymer film is analyzed by a colorimeter (e.g. Minolta CS 100), which yields Yxy values. Perhaps the most useful information found through colorimetric analysis is the relative lumina nce (%Y). Here the measured luminance value (Y) is taken relative to a standard wh ite illuminant (D50, 5000K) in a dark booth designed to exclude external light. Calcul ating the difference between %Y values measured at various applied potentials yields a measure of the contrast ratio that takes into account all wavelengths of the visibl e spectrum and the nonlinear response of the human eye.


49 CHAPTER 3 PATTERNING OF REFLECTIVE ECDS USING SHADOW MASKS The broad spectral absorbance of electroact ive polymers, which can range from the UV through the visible, NIR, mid-IR, far IR , and into the microwave region by accessing various oxidation states, ha s resulted in the need to modify the definition of electrochromism to include this multi-spectral window for photon energies.80, 86 Our group has developed a number of EC polym ers based on polythiophenes and polypyrroles with band gaps ranging from ca. 1.0 eV (1200 nm) to over 3.0 eV (400 nm), effectively spanning the full visible region.29, 30, 41, 42, 50 Of these, the PXDOTs allow electrochemically stable ECDs that exhibit fast switching times and high contrast ratios in the visible and NIR range. PE DOT, PProDOT and the dimethyl derivative of PProDOT (PProDOT-(Me)2) have emerged as promising candidates for polymeric ECDs. As an example, PProDOT-(Me)2 exhibits a high transmittance c ontrast in the visible region (350-800nm) with a %T = 78% at max=578 nm and a colorimetrically measured luminance contrast of 60%. Electrochr omic switching of films of PProDOT-(Me)2 tuned for optimal contrast occurs in less than 400ms.30 Recently, we and others have utilized a dual-polymer configuration for the construction of reflective ECDs.90-92 The outward facing active electrode is convenient for obtaining reflectance characterizations and for modifying the optical response from a surface. In initial studies, gold-coated Mylar with parallel slits cut to allow for ion transport between the polymer film and a c ounter electrode hidden beneath were used. These reflective ECDs exhibit high EC contrast s in the visible, NIR, and mid-IR regions


50 of 55%, 80% and 50% (specular response) , respectively, an d maintain their electroactivity for tens of t housands of redox switching cycles.92 This type of device construction is applicable to thin and flexible electrochromic platforms.81 This chapter introduces the use of highly porous metallized membranes in patterned reflective ECDs that switch rapidly. Figure 31 illustrates the device design as initially developed by Bennett et al.90 and Chandrasekhar,91 and is representative of other reflective type ECDs comprising different me tals on porous substrates. Gold-patterned porous electrodes were prepared by means of a shutter mask during a metal vapor deposition process allowing the deposition and independent addressing of more than one active electrode. With PEDOT, PProDOT, or PProDOT-(Me)2 as the active EC materials, switching times were 0.1 to 0.2 seconds (5-10 Hz) to achieve full EC contrast. During the redox switching process, charge-carrying ions easily migrate from the polymer through the thin porous membrane, giving a short diffu sion distance between electrodes. Nickel patterned porous electrodes were also used as an alternative to gold since nickel has a neutral gray color and it proved effective as a non-reacting conducto r in the potential window that the EC polymers are deposited and switched. Au-plastic substrate Polymer CE Gel electrolyte Porous membrane a Au contacts Electroactive polymer layer Support Transparent Window (a) (b) Figure 3-1. (a) Schematic representation of a refl ective type electrochromic device (ECD) using a porous membrane electrode . (b) Cross section of the ECD.


51 Electrode Patterning In order to prepare porous, yet highly reflective, metal contacts for the electroactive polymers, vapor deposition was used th rough shadow masks onto polycarbonate membranes. Figures 3-2a and b show tw o examples of gold patterned electrodes demonstrating the ease with which pixels of 1 x 1 cm could be attained. It is important to note that the pores within the membrane are not clogged during th e deposition process because the film thickness (50 nm) is small compared to the pore size (10 m). Figure 32c demonstrates this with a reflective op tical micrograph of a membrane after the metallization process, clearly showing that th e pores (black holes) are preserved. Figure 3-2d shows a photograph of the glass substrat e that the membrane had been placed on during the gold deposition proce ss. It is covered by the resi due, or ghost image, of the gold which passed through th e pores of the membrane. The gold surface has a total reflectance (%R) of ~90% throughout the NIR region with a measured diffuse reflectance (%Rd) of ~70% and a specular reflectance (%Rs) of ~20%. Although the PC substrate is quite transmissive in this region (~50%), th e transmissivity drops to ~3% following the gold deposition. The films are conducting, with a surface resistivity of R ~ 5 /sq as measured by a two point probe conductivity meter. The resistance through the goldcoated porous membrane is >20,000 .


52 ab d c 100 m Figure 3-2. (a) Two gold pi xels (1.5 x 2 cm) patterned on a polycarbonate (PC) membrane. (b) A 2 x 2 gold pattern (2 x 2 cm) on PC. (c) Magnification (80x) of the metallized membrane to show the unfilled pores. (d) Image of the pattern on the glass backing plate us ed during the evaporation process indicating that the gold passes thr ough the membrane during deposition. Reflective ECDs from Micropo rous Gold Electrodes Reflective type ECDs comprising dioxythi ophene based polymers (PXDOTs) were built using patterned microporous electrodes show n in Figure 3-2. Potentiostatic methods, where a constant potential is applied in a three-electrode cell configuration until the desired amount of charge is passed, ar e the most convenient techniques for the electrochemical deposition of PXDOTs onto metallized membranes. Resulting films are well-adhered and have high integrity. PBEDOT-B(OC12H25)2 was deposited more effectively using a potentiodynamic multisweep method between –0.6V and 0.9V vs. Fc/Fc+. The metallized membranes have low surface resistivity values (5 /sq), which minimize the effect of the ohmic drop along the electrode surface. The polymers are doped/dedoped easily on these highly conducting surfaces. In all cases the electrochemical behavior of the EC polym ers were found to be stable upon several thousands of switches as reported previously.77


53 Device Design and Construction In order to obtain good performance of the ECDs in terms of co lor contrast, it is necessary to pair the front active layer with a counter electrode (CE) containing a higher amount of electroactive polymer. This combinati on allows the CE to serve as an electron sink and ion storage layer for the active layer so that the electrochemical properties of the CE do not limit the optical contrast of the active layer. The identity of the polymer deposited onto the CE does a ffect the kinetic performance of the devices (for example, PEDOT switches more slowly than PProDOT-(Me)2, yet has no influence on the steadystate properties of th e devices (such as %R) in the visible-NIR regions. Table 3-1 lists the components used in six PXDOT based de vices (D1-D6) examined in this study. D1 refers to results from a device that utilized a slitted active layer previously published by our group.92 D2, D3/D4, and D5 refer to devices constructed with PEDOT, PProDOT and PProDOT-(Me)2 as the active layers, respectively. Finally, D6 represents a dual-polymer PEDOT and PBEDOT-B(OC12H25)2 reflective device with a 2 x 2 pixel configuration. Table 3-1. Components used in construction of the reflective electrochromic devices. Device # Active layer Working Electro de Counter Electrode Environment D1 PProDOT-(Me)2 Slitted PEDOT Air D2 PEDOT Porous PEDOT Air D3 PProDOT Porous PEDOT Air D4 PProDOT Porous PProDOT Air D5 PProDOT-(Me)2 Porous PProDOT-(Me)2 Air D5-inert PProDOT-(Me)2 Porous PProDOT-(Me)2 Argon D6 PEDOT and PBEDOT-B(OR)2 Porous PEDOT Air


54 Spectroelectrochemical Characterization Reflectance spectroelectrochemistry gives us the ability to probe the attenuation of reflectance from the metal electrode induced by the electroactive polymer in both the visible and near infrared regions of the electromagnetic spectrum. Neutral PEDOT, PProDOT, and PProDOT-(Me)2 have similar electronic band gaps and max values, giving them very similar colors. When these polymers are held in their neutral forms on the devices, they are difficult to distinguish. This behavior is evident in the visible region of the R results of Figure 3-3a (here, R = %Rneutral %Roxidized, is the reflectivity contrast) which presents data for devices D2 (curve A), D3 (curve B) and D5 (curve C). At the same time, there is a difference in the NIR reflectivity contrast among the samples. PProDOT and PProDOT-(Me)2 devices D3 and D5 have a higher contrast in the NIR ( Rnir reaching 70%) relative to the PEDOT device D2. This improvement may be attributed to a more open morphology in the ProDOTs induced by the more sterically demanding rings, which allows higher levels of doping and, thus, higher contrast ratios.27 Photographs of a D5 type devi ce are shown in Figure 3-3b which demonstrate EC switching between a dark-blue (neutral polymer) absorbing state, and a very transmissive (oxidized polymer) state, revealing the highly reflective gold surface. Dramatic improvements in switching speed were observed in these second generation devices relative to the D1 type slitted de vices simply by modifying the nature of the conducting substrate. The 3 cm2 device pictured switches between the absorptive and reflective states in sub-second time frames with a 95% optical switch a ttainable in 200 ms (discussed in detail in the next sectio n).The optical and electrochemical switching


55 properties for these devices are presented in Table 3-2. The PProDOT-(Me)2 based devices, D1 and D5, possess higher %R values in the visible and 400600800100012001400160018002000 -60 -40 -20 0 20 40 60 80 100 (C) (B) (A) Visible NIR %R /nm (a) (b) Figure 3-3. (a) Refl ectivity contrast ( R = %Rneutral %Roxidized) spectra of D2 PEDOT (A), D3 PProDOT (B), and D5 PProDOT-(Me)2 (C) devices. (b) The two photographs represent (left) the oxidized and (right) th e neutral appearance of the active layer. NIR regions than the PEDOT and PProDOT based devices, D2 and D3. The similarity in the reflectivity contrast values observed for D1 and D5 is reassuring because two different substrates were used for the active layer, specifically Au on slitted Mylar (used for D1) and an Au-coated porous polycarbonate membrane (used for D5). The enhanced reflectance contrast of the PProDOT-(Me)2 device, relative to the PEDOT and unsubustituted PProDOT device, is consistent with earlier studies showing the dimethyl derivitization provides e nhanced EC contrast in the polymer film. One D5 type device (D5-inert in Table 3-1) was also constructed in a glove box to ensure the absence of any oxygen and water. The spectroelectrochemistry of this PProDOT-(Me)2 based device, shown in Figure 3-4, e xhibits the same optical properties as the device built on the desktop, i.e. %RVIS = 55% and %RNIR=70%. When reduced


56 Table 3-2. Optical reflectivity contrast in the visible ( %RVIS) and the NIR range ( %RNIR) for the devices D1-D5. Also given are the composite coloration efficiency and switching time values. Device # D1 D2 D3/D4 D5/D5-inert VIS contrast ( %RVIS) 55% 40% 40% 55% Rmax) (nm) 600 573 534 549 NIR contrast ( %RNIR) 80% 40% 70% 70% Rmax) (nm) ~1750 1265 1260 1540 * (cm2 C-1) N/A 259 372 607 Switching Time* (ms) 3000 1050 400 / 200 100 / 90 * Data taken at 95% of the full % R. (curve a), the spectrum exhibits a sharp absorption peak at =620nm (minimum of %RVIS) corresponding to the * transition of the polymer. Upon initial oxidation, at voltages between –1.0V and –0.4V (curves a-d), there is little change in the color of the device ( %RVIS is low), whereas the 750-1200 nm NIR absorption increases. This band is attributed to absorption by the upper pol aron band of the lightly-doped polymer.31 The visible absorption then decreases concurren tly with further increase of NIR absorption due to the bipolaron band and free charge ca rriers (curves d-g). The first change in reflectivity in the visible window appears at -0.2V ( %RVIS=5%, compared to %RNIR=50%). We speculate that when the polymer is in its neutral state, the NIR light fully penetrates through the polymer layer wher eas the visible light is strongly absorbed. When the polymer is partially oxidized, the reflected NIR light is more sensitive to the optical changes that occur close to the elec trode surface than the reflected visible light. As a result, the absorption of the charge carriers in the NIR region increases (%R decreases) with minimal depletion of the * absorption in the visible region. This


57 effect has also been reported by Chandrasekhar et al.81 who attributed this particular behavior to a micrometer scale morphology transition of the polymer during the redox switching which alters the scattering compone nt of the reflected light at comparable wavelengths (1~3µm). 400600800100012001400160018002000 0 20 40 60 80 100 (a-g) f e c b d a gVisible NIR %R/nm Figure 3-4. Spectroelectroc hemistry of a PProDOT-(Me)2 active layer in a D5-inert type reflective device. (a) –0.8V, (b) –0.6V , (c) –0.4V, (d) –0.2V, (e) 0.0V, (f) +0.2V, and (g) +0.4V. Electrochromic Switching and Stability The use of a porous electrode as the ac tive layer affords a homogeneous color change of the EC film as the device switche s, as opposed to that observed for the device D1 in which the color change initiates at the slits and moves laterally across the surface of the electrode. The highly porous membrane s allow the devices to be switched quite rapidly. In potential-step experiments performed on D4 shown in Figure 3-5a, the switching time is set every 1 second between neutral and oxidized states. The optical switch is fast and fully reversible. Examini ng a single transition mo re closely (Figure 35b) shows that the switching time between th ese two extreme redox states is ~200 ms. Table 3-2 lists the switching times for the diffe rent devices. In general, using the porous


58 membrane electrodes, the switching times are sub-second. Using the slitted electrode with 1-2 mm separation, a few seconds is requ ired to attain the full transition. By replacing the slitted gold-Mylar electrode by a porous metallized substrate where ion diffusion lengths are minimized, a substantia l improvement of the device’s switching speed is obtained. 0246810 20 40 60 80 100 %R1 sec switchingTime (sec) 20 40 60 80 t ~ 200ms (558nm)%R Time (sec) (a) (b) Figure 3-5. (a) Temporal change in %R (1540 nm) during electrochromic switching of a D3 type reflective device between –1V and +1V every 1 second. (b) A single transition illustrating the switching time of the same device (-1V to +1V, =558 nm). We investigated device stability by switc hing D5-inert (constructed and sealed under argon) 180,000 times between –1V and +1 V every 3 seconds while monitoring the %R at 1540nm as shown in Figure 3-6. The in itial contrast of th e device is 75% and throughout the experiment, the oxidized form of the active layer gives a stable reflectivity of %RNIR = 20%. At the same time, the reflectivity of the neutral form of the active layer slowly decreased from 95% to 89%. Follo wing the completion of the 180,000 switches, the device was held at a constant voltage fo r an extended period of time and the initial contrast value was recovered in ca. one minute. Subsequently beginning the multiple


59 switching process again shows this loss of c ontrast to be permanent as the contrast quickly drops to the value observed before the applied voltage annealing. By slowing the switching speed to > 3s, the full original contrast could be retained. If the device contains air (oxygen and water) as in D5, the decrease in reflectance contrast occurs at a faster rate, with %R=15% after only 35,000 switches. Speculating on this issue, we note that this is a kinetic phenomenon and not an irreve rsible oxidation of th e neutral polymer in air. Possible explanations include a decrease in the ionic conductivity in the cell due to slow evaporation of the so lvent and a reorganization of the polymer film morphology slowing ion movement in the cell. 020406080100120140160180 0 10 20 30 40 50 60 70 80 90 100 %R (1540 nm) Oxidized Neutral Thousands of Switches Figure 3-6. Long-term switching stability of a D5-inert type device sw itching between –1V and +1V every 3 seconds. Composite Coloration Efficiency (CCE) Coloration efficiency is an efficient a nd practical tool to measure the power requirements of a device. In e ssence, it determines the amount of optical density change


60 ( OD) induced as a function of the inj ected/ejected electronic charge (Qd) during a potential step, i.e. the amount of charge necessary to produce the optical change in the polymer.22 This concept has been used in electrochromic studies to compare ECDs containing different materials.28, 118 OD is directly related to the amount of the doping/dedoping charge Qd) by the equation: dQ OD where (cm2/C) is the coloration efficiency at a given . Our group has developed a practical method for measuring coloration efficiency, termed com posite coloration efficiency (CCE), where the OD during an electrochromic switch with a pre-determined OD or color change desired for an application is used.27 The CCE experiment empl oyed here consisted of a series of potentiostatic steps from the neutra l state (-1V) to the oxidized state (+1V) while both the charge passing through the ce ll and the reflectivity are monitored as was shown in Figure 3-5b. While these CCE experime nts have previously been performed on transmissive type devices, for this study we obtained values for reflective devices. To the best of our knowledge, these are the first coloration e fficiency experiments performed with reflected light as opposed to transmitted light. For comparison, the coloration efficiencies ( calculated for a 95% optical change and the associated switching times are listed in Table 3-2. The OD=95% switching times of the PXDOT derivatives decrease from PEDOT (1.05s), to PProDOT (200ms), to PProDOT-(Me)2 (90ms) and the CCE values of 259, 372, and 607 cm2 C-1, respectively, increase. These values are consistent with the transmission/absorpti on optical contrast ranking as published previously30 demonstrating the utility of this method for device studies. It is important to note here that due to the double pass of the light through the polymer layer, the reflective CCE is considered as the transmission /absorption CCE with double the polymer


61 thickness. PProDOT-(Me)2 exhibits a full contrast in ~200 ms in a porous type device, as shown in Figure 3-5b. By using the thin por ous membranes, the active layer and counter electrode distance is relatively small (50-100 m) reducing ion diffusion lengths and shortening switching times. This device platfo rm serves as an improved configuration for a reflective electrochromic cell. Open Circuit Memory One of the benefits of using an electrochro mic material in a display as opposed to a light emitting material is the EC memory effect . As explained earlier in Chapter 1, open circuit memory (also called optical memory) is defined as the time an electrochromic material retains its absorption state after re moving the electric fiel d. After setting the device in one color state and removing the electri c field, it should retain that color with no further current required; thus giving the de vice an open-circuit memory. In reality, ECDs require small refreshing charges in order to maintain the charge state because side reactions change the desired color. Figure 3-7 illustrates the variation of the reflectivity (%R) in the visible (a) and NI R (b) regions for both the oxidized and neutral states of PProDOT-(Me)2 as the active layer fo r a device constructed a nd sealed under argon (D5inert). In this experiment, we applied a pulse (-1V or +1V for 1 second) and then held the cell in an open-circuit conditi on for 300 seconds while the re flectivity was monitored as a function of time. The change in %R tends to move the device to an equilibrium state and represents a loss of memory, i.e. the ability of the device to retain the reflectivity imposed by the pulse. We observe that the reflectivity of the neutral state is highly stable in the visible region (Figure 3-7a) a nd the oxidized state exhibited a dt R % dVIS = 0.4 % min-1 loss while being held at open circuit. The shor t 1 s pulse (+1V) fully recovers the initial


62 %R. This behavior is oppos ite in the NIR region. A dt R % dNIR = 1 % min-1 at open circuit is recorded when the device is set to its neutral state. However, the reflectivity loss is easily regained by a new voltage pulse. This study reveals that the sealed device can exhibit its full reflective propert ies with a brief supply of en ergy. Devices constructed and tested under ambient atmosphere exhibit simila r behavior with a more rapid decrease in NIR reflectivity contrast ( dt R % dNIR> 1 % min-1); again the 1 second: 1 Volt pulse recovers the initial electrochromic states. 051015202530 20 30 40 50 60 70 80 90 100 (Neutral) (Oxidized)p u lse %R (558 nm) Time (min) 051015202530 20 30 40 50 60 70 80 90 100 (Oxidized) (neutral)puls e %R (1540 nm) Time (min) (a) (b) Figure 3-7. Open circuit memory of a D5-ine rt type device monitored by singlewavelength reflectance spectroscopy. A 1V pulse is applied for 1 second every 300 seconds to recover the initial reflectance. (a) Visible memory at 558 nm, and (b) NIR memory at 1540 nm. Energy Consumption To establish the energy consumption of these devices, we compared the power necessary to switch a porous type ECD (D5) to a slitted type ECD (D1) with the slits separated by 2-3 mm. Table 3-3 contains the electrical characteristics of D1 and D5 use to calculate the energy (E) per unit area of ECD necessary to switch from one redox state


63 to another. The energy is given by dt ) t ( i V E, where V is the pulse (1V) applied to switch the device and i(t) is the time dependent curre nt. Under these conditions, ) t ( Q V dt ) t ( i V E with Q(t) being the charge pa ssed during the pulse. It is evident that using the porous electrodes substantially reduces the energy consumption when compared to an active layer that has been prepared with slits. These data allow us to theoretically estimate the energy require ments of a large area ECD assuming the iR losses are scalable to large areas. We consider a 1 m2 surface active device (mass of ca. 600g/m2) connected to a state-of-the-a rt 1 kg Li-battery (400 kJ kg-1, 111 W kg-1).119, 120A device like D5 will hold 8000 hours (1 year ) in any single state. Then, a 1 m2 ECD constructed as D1 and D5 will switch 26, 000 and 60,000 times, respectively, using this battery. If switched 500 times during a day and held between switches in either one redox state, the 1kg battery can opera te a D5 type device for 50 da ys. As another example, we consider a device operation as described in the previous section where the ECD is refreshed with 1 second: 1 Volt pulses every 300 seconds to maintain either bleached or colored state. The 1 kg battery can then refresh the same D5-type device (A = 1 m2) 600,000 times (operation time of ~2100 days) w ith an energy consumption of only 0.67 J/pulse. For smaller devices, we consider operation of a 4 cm x 4 cm ECD display running on a light-weight (1.5 g) button-type alkaline battery (1.5 V, 100 mAh). Using this battery, the D5-type devi ce will switch ~34,000 times. These practical considerations suggest this reflective device platform based on PXDOT polymers can be considered for numerous applications.


64 Table 3-3. Energy consumption data for D1 and D5 type devices. D1 Slitted-type device D5 Porous-membrane device V 1 V 1 V Charge, Q 15 mC 2 mC Area 10 cm2 3 cm2 Pulse time, t 10 sec 1 sec Energy, E/area 1.5 mJ/cm2 0.67 mJ/cm2 Pixelated Lateral ECDs The adaptability of the dual-polymer con cept previously published for transmissive ECDs77 to the lateral reflective ECDs was e xplored. As a demons tration, we have developed a device where a high contrast active area and color matching concepts are simultaneously used (D6). Specifically, a pa tterned porous substrate composed of the 2 x 2 gold-square pattern originally shown in Figure 3-2b, is coated by two different polymers PEDOT and PBEDOT-B(OC12H25)2. In order to address each set of pixels individually, gold contact tr aces are also deposited on th e porous membranes. A method to hide these traces on the back of substrat es will be presented later in the “Back-side Contacts for Patterned ECDs” section. The electrodeposition of each polymer was first performed on the separate electrodes and the films were reduced in a 0.1M TBAPF6/propylene carbonate solution to provide the colors evident in Figure 3-8a. A Pt flag electr ode was shared as the counter electrode. At a bias of –1 V vs. Ag0, PEDOT is blue while PBEDOT-B(OC12H25)2 is red. When the bias is reversed and both polyme rs are fully oxidized at 1 V, both polymers switch to a highly transmissive state, exposing the reflectiv e gold surface. An ECD using this pixelated electrode was constructed in a similar manner as previously described, using a PEDOT counter electrode. As show n in Figure 3-8b, when both polymers are


65 reduced they reveal the colors of the two pol ymers so that the device exhibits an optical surface contrast. When they are oxidized, they are visibly transmissive presenting a uniform shiny gold surface. The PXDOT-bas ed electrochromic polymer family (developed in our group) offers many possibi lities to use dual-polymer lateral reflective ECDs where the materials can be matched for multi-colored displays applications. S O O n S O O S OO O R n O R PBEDOT-B(OR)2PEDOT S O O n S O O n S O O S OO O R n O R PBEDOT-B(OR)2PEDOT (a) (b) Figure 3-8. (a) Photographs of EC switching of PEDOT and PBEDOT-B(OR)2 on a 2x2 pixel gold/membrane electrode. Left: Both polymers in their neutral (colored) states. Right: Polymers in their oxidized (bleached) states. (b) A 2 X 2 pixels device (D6 type) using the patterned el ectrodes described above. Left: Both polymers reduced (colored). Right: Both polymers oxidized (bleached). Reflective ECDs from Micro porous Nickel Electrodes In an attempt to replace gol d with another compatible electrode material, several other metals such as nickel, copper, titanium, palladium, and platinum were investigated.


66 To be used in reflective type ECD applic ations, the selected metal should be highly conducting, inert in the potent ial window that EC polymers are deposited and switched, and reasonably reflective to allow spectrosc opic characterizations. Table 3-4 lists the volume resistivity values of these metals along with their temperature requirements for vapor deposition. Since polymer membranes de grade at high temperatures during metal vapor deposition process (~150 0C for polycarbonate membrane), it is important to use a metal which evaporates at low temperatures. Since platinum, titanium, and palladium require high temperatures to vaporize, they were not considered in this work. In addition, titanium yields low reflectance values in the spectral region of interest (~40% throughout visible and NIR). Low oxidation potential and its unappeal ing color are major limitations for copper to be used for ECD applications. Table 3-4. Metal candidates to be used in reflective ECD applications. Melt Temp (oC) Vaporization Temp (oC) Heat of Vaporization (kJ/mole) Resistivity ( -cm) Au 1064.2 2808 324.4 2.125 Ni 1455 2732 377.5 6.844 Cu 1083 2562 300.5 1.673 Pt 1768.4 3825 510.4 9.85 Pd 1554.9 3167 393.3 9.93 Ti 1688 3277 425.2 47.8 Here we demonstrate the use of nickel coat ed porous electrodes to build reflective type ECDs. Nickel was eva porated (50 nm) on a microporous polycarbonate membrane through a shadow mask under high vacuum (10-7 Torr). Metallized membranes have surface resistivity values of ~200 /sq. Electrochemical deposition of PEDOT on nickel electrodes is carried out potentiodynamically using a multi-sweep method between –0.6V and 1.5V vs. Ag0 as shown in Figure 3-9a. As a control experiment, PEDOT was deposited on a same size gold coated Kapton electrode which yielde d identical oxidation


67 and reduction potentials and similar current densities. Using PEDOT coated nickel electrode as the active layer, a reflective type ECD is built according to the device scheme given in Figure 3-1a. Figure 3-9b shows the EC switc hing of this device between neutral and oxidized states of PEDOT. - -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 I (mA/cm2)E (V) vs. Ag0 (a) 1.0 V (b) Figure 3-9. a) Accumulative deposition of PEDOT on a nickel coated microporous polycarbonate membrane (E lectrode area = 1.7 cm2). b) EC switching of a PEDOT device comprising nickel electrode s. Left: -1.0V, right: +1.0V. Back-Side Electrical Contacts for Patterned ECDs Patterning of electrodes for electronic device s is essential for fabrication of finestructured electronic circuits, independen tly addressed displa y devices with high resolution, and for devices which require sepa ration of adjacent el ectrodes. Conventional


68 direct writing methods such as photolithogra phy are widely used due to their nanometer scale resolution values, but suffer from multi-st ep preparation procedures and high cost. Soft lithography techniques such as micro-contact printing ( CP)101 employ molds and masks and have proven useful since they ar e non-reactive and require mild processing conditions. Other methods include, but are not limited to, metal vapor deposition through shadow masks, line patterning,108 screen printing,110 and inkjet printing.106 Depending on the device application and the t ype of substrate used, above methods (or combinations of them) are employed to fabricate electrodes and electrical contacts which address them. Electrical contact to electroma gnetically active devices, such as integrated electronics, electroluminescent, photovoltaic, electrochromic , and other devices, is typically provided using electrically conductive tr aces which connect electrode s to conductive structures disposed on the same side of the device. These traces are usually isotropically conducting; i.e., they conduct el ectricity in all directions through the material. Another type of contact pad is an an isotropic z-axis conductor wh ich allows conduction in the direction perpendicular to the electrode material.121 This is valuable for interconnection of materials through vertically integrated systems and th ree-dimensional electronics where the conduction through th e film thickness is desire d without any lateral (x-y plane) electrical shorting. Z-axis films are obtai ned by mixing conducting particles or clusters (usually metal) with a polyme r matrix with an insufficien t amount of particles in the x-y plane to form a conducting network. When th e matrix is squeezed, randomly distributed metal particles align to form a conducting path perpendicular to the plane. Nickel, silver, KTiPO4, and some other metal oxides were studied in the literature as z-axis conductive fillers to provide resistance values of ~1-100 .121-123


69 Conductive traces and bond pa ds can significantly dimini sh the available area for active devices. Moreover, such arrangements can introdu ce performance limitations, as well as affect the appearance of the device fo r display applications, such as for certain electrochromic display devices. As an exam ple, the appearance of electrical contact traces between independently addressed pixe ls of the dual colored ECD shown in Figure 3-8b revealed the need for a technique to br ing pixels closer (mak ing invisible traces) without compromising conductivity. Electrode Preparation A method is developed to prepare patterned electrodes on porous substrates such as ion track etched membranes, prefilters, and filter papers where the contacts to address these electrodes are hidden on the back of th e substrates. This is illustrated in Figure 310a for a gold patterned track-etched membrane and a fiber-like filter paper. These electrodes can be considered for use in a vari ety of electronic device applications such as electrochromic display devices , electroluminescent devices, thin film transistors, photovoltaic devices, and other ve rtically integrated electr onic devices which require a conducting electrode material to operate. In th is concept, the front face is the active side which includes the desired pattern and the b ack side includes the co ntacts to address the patterned regions. The first type porous substr ate (Figure 3-10a, left) is an ion track etched polycarbonate membrane with well-defined pore sizes (10 m) having a nominal pore density of 105 pores/cm2 (7.6% porosity). Metallizati on of both surfaces results in partial filling of the membrane pores with gold clusters and a high electrical conductivity between the front electrode and the back c ontact is achieved. The resistance between a front electrode and the back contact thr ough the track-etched porous substrate is 10-4


70 ohms for a substrate thickness of about 100 µm and an area of about 1 cm2. As such, the series resistance from contact ing the front-side electrodes from the back-side of the substrate is negligible as compared to conve ntional front-side contac ts. The pores within the membrane are not clogged during the de position process becau se the gold thickness (~60 nm) is small compared to the pore si ze. Figure 3-10b demonstrates this with a reflective optical micrograph of a membrane after the metallization process, clearly showing that the majority of por es (black holes) are preserved. Figure 3-10. Back-side addre ssed electrodes using porous substrates. a) Schematic representations of an ion tr ack etched membrane with well-defined pores (left) and a fiber-like porous membrane (right). Gold is deposited on both front and back sides of the membrane. b) Reflec tive optical micrograph of a double-side gold coated track-etched membrane. Black holes represent the unfilled pores. c) Reflective optical micrograph of a double-side gold coated laboratory filter paper. Electrical conductivity betwee n the top and bottom gold layers is induced using a PEDOT/PSS film proce ssed from an aqueous dispersion. The second type of porous substrates th at have been used are prefilters and laboratory filter papers. These s ubstrates have fiber-like stru ctures (Figure 3-10c) and are porous without well-defined pores. Gold deposition on both sides of this type of substrates results in electrically insulated fr ont and back sides where the metal penetrates a minimal amount into the membrane. Electri cal conductivity can th en be induced at (c) 50 m (b)Front-side Gold layer Track-Etched Membrane Partially filled pores Back-side Contacts Front-side Gold layer Porous Membrane PEDOT-PSS (a)


71 desired areas by introducing a solution or melt processable conductor into the porous filter between the metal layers as shown in Figure 3-10a, right. This material will perform as a z-axis conductiv e filler providing electrical cont act through the thickness of the porous membrane without having to short pa tterned pixels. In one example, we have used a commercially available, highly conducting, and solution processable polymer dispersion, PEDOT/PSS, to br idge the front and back sides of these substrates electrically. The use of PEDOT /PSS as a transparent electro de material to build allorganic ECDs will be discussed later in Chapter 5. PEDOT/PSS is applied onto the gold coated, 1 mm thick porous substrates (application area of 1 cm2) by drop casting to yield resistance values of as low as 10-3 ohms between the front and the backsides. The aqueous solution of this conducting polymer diffuses into the membrane and forms a conducting network between tw o sides after drying. Once dried, PEDOT/PSS is no longer soluble in common solvents and doe s not return to the non-conducting form at ambient conditions. The region specific conduc ting polymer bridges can be done prior to or after the gold deposition. Ot her solution processing methods such as ink-jet printing and spray printing can also be used to apply PEDOT/PSS onto these substrates. Reflective ECDs As a demonstration of the applicability of the back-side contacts method in organic devices, reflective type ECDs from electrochr omic EC polymers are constructed. Figure 3-11 shows a schematic represen tation of a reflective type ECD along with its operation mechanism employing PProDOT-(Me)2 as the active EC polymer. PProDOT-(Me)2 was electrochemically polymerized on a gold/membra ne electrode as the active layer (layer ii in Figure 3-11) and on a gold coated non-porous plastic substrate as the ion storage layer (layer vii in Figure 3-11). A three-el ectrode cell was utilized to deposit the polymer from


72 a 0.01M monomer electrolyte so lution with the gold coated electrodes being the working electrode, a platinum flag as the counter electrode and a silver wire as the pseudoreference. Following a layer-by-layer configur ation, ECDs were assembled employing an outward facing device scheme previously used for devices with fr ont contacts. PProDOT(Me)2 on the top electrode (layer ii ) is in its neutral (colored ) state as assembled. When a positive voltage ca. +1.0V is applied between th e back-side contacts (layer v) and the counter electrode (layer viii), PProDOT-(Me)2 is oxidized and the doping anions move upwards through the ion permeable membrane in order to balance the polymer’s positive charge. Figure 3-11. A reflective type ECD scheme using back-site addressed electrodes. iOptically transparent window, iiPProDOT-(Me)2, iiiAu, ivPorous membrane, vBack-side gold contact, viAn opaque porous separator soaked in gel electrolyte, viiPolymer counter electrode, viiiAu/plastic. Light absorption of the top polymer layer can be modulated by applying 1V between the layers v and viii. In situ reflectance spectroel ectrochemistry is carried out to monitor the optical changes of the top polymer layer as it is sw itched from -1V to +1V as illustrated by the results in Figure 3-12. Using an integr ating sphere mounted to an UV-Vis-NIR spectrophotometer, a total reflectance spectrum (specular + diffusive) is recorded at each applied voltage. When a nega tive voltage (e.g. –1.0V) is applied to the device, the -1 V viii vii vi v iv iii ii i +1 V Computer driven voltage source A


73 polymer is in its neutral state and it appears deep blue as PProDOT-(Me)2 is cathodically coloring. The spectrum at -1.0V (Figure 3-12, cu rve a) exhibits a sharp absorption peak at =620nm (minimum of %R) corresponding to the * transition of the polymer. As the voltage is increased stepwise to +1.0V, the visible absorption decreases and a new absorption band in the NIR region (700-1200 nm ) is observed. When fully oxidized, the polymer switches to its bleached state; therefore the gold la yer beneath the polymer layer becomes fully observable to the eye. The device shows a high reflectance contrast of %R ~ 60% in the visible region and up to %R = 75% in the NIR region. When a large magnitude potential step from +1V to -1V is applied, the device switches from its oxidized state to neutral state in less than 0.5 seconds. When switched from neutral state to oxidized state, time is longer (~1.2 sec onds). We attribute this discrepancy in electrochromic switching to the hi gher resistance of PProDOT-(Me)2 in its neutral state where the polymer is insulating. 400600800100012001400 0 20 40 60 80 100 g-i a-f e i a a-c d-i%RWavelengh (nm) Figure 3-12. In-situ reflectance spect roelectrochemistry of a PProDOT-(Me)2 ECD. Applied voltages: (a) -1.0V, (b)-0.8V, (c) -0.6V, (d) -0.2V, (e) 0 V, (f) 0.2V, (g) 0.4V, (h) 0.7V, and (i) 1.0V.


74 Long term EC switching is studied by employing singl e-wavelength spectroscopy where the device is switched between two ex treme states (-1.0V and +1.0V) every 3 seconds while monitoring the %R change at 600 nm . The devi ce was switched over 100,000 times with less than 15% contrast loss. Throughout th e experiment, the oxidized form of the active layer gives a stable reflectivity of %R = 62%. At the same time, the reflectivity of the neutral form slowly (and irreversibly) increases which is the main cause for the optical contrast loss. It should be noted that this de vice was constructed and switched with air exposure and no extreme encapsulation. It is expected that by constructing and encapsulating the device in inert atmosphere, lifetimes of >106 cycles would be possible. Digit-Display ECD Finally, a numeric display electrochromic device was designed and assembled to demonstrate the independent addressing of patte rned electrodes with back-side contacts. A visibly transparent track-etched polycar bonate membrane was used as the porous substrate material. The front side of the me mbrane was covered with gold through a mask shown in Figure 3-13a using a high vacuum metal vapor deposition process. During the metallization process, the membrane was sandwiched between a clean piece of copper coated epoxy and the shutter mask allowi ng the patterning of the membrane surface. Seven electrically independent gold pixels were produced. Gold electrical contacts were then deposited on the back of these pixe ls using the mask shown in Figure 3-13b. PProDOT-(Me)2 was electrochemically deposited on each pixel as well as on a gold coated plastic (counter electrode). The numer ic display ECD was assembled to form the ECD structure according to the device scheme previously described in Figure 3-11. The counter electrode was first pl aced as the bottom layer, w ith the polymer coated side


75 facing up. A thin layer of gel electrol yte was homogenously applied on the counter electrode. The patterned membrane was then pl aced on the top, the front side (pixels) facing up. Finally, an optically transparent plas tic was used to cover the device. A voltage was applied between the back-side contacts a nd the counter electrode where each pixel’s voltage is controlled separately through a D/A converter interface moderated by a virtual instrument program written in National In strument’s LabView software. The counter electrode was shared for all th e active pixels and was grounded. (a) (b) (c) Figure 3-13. Machine-cut masks used to patter n gold on front (a) and back (b) sides of porous membranes. c) Photograph of a 7-pixel electrochromic numeric display device showing the number “5”. Device dimensions: 3cm x 5cm. Figure 3-13c shows a photograph of a nume rical display ECD along with its backside contacts. Specifically, it shows the numbe r “5” where five of these pixels are blue colored (neutral state, applied voltage: -1.0 V) and the remaining two are bleached to show the gold color beneath (oxidized stat e, applied voltage: +1.0V). Pixels are independently addressed and their contacts to the voltage source we re all made using back-side contacts. The high color contrast achieved is because of the difference in absorptivity of the gold surface and the elec trochromic polymer layer. Numbers “0” to “9” can easily be obtained by proper assignm ent of pixel voltages. EC switching from one number to another occurs in less than a second.


76 Conclusions In conclusion, this chapter presents the design of reflective platforms in which one or two electrochromic polymer(s) cover a re flective gold or nickel surface mounted onto a uniformly porous membrane using pattern ing techniques. Using a sandwich-type configuration, the electro-active platform was paired to a polymeric counter-electrode in order to realize reflective ECDs. The alkylenedioxythiophene-based polymers are suitable materials for broadband electr ochromic applications. Our goal was to utilize these materials on a porous ECD architecture in or der to attain fast switching, high switching stability, and have low energy consumption for maintaining a specific color state. We have also demonstrated patterning of porous substrates with electrical contacts from the back side of substrates. Back-sid e contact method permits increased density and more design flexibility for display type de vices as compared to conventional front-side contact techniques. Devices containing metallized porous substrates can provide a significant performance improvement over conventional non-porous substrates. Any device in which it is desired to have a se ries of patterned elec trodes on one substrate surface and contacts on the back substrate su rface can benefit from this method. These devices include electrochromic display devi ces as noted above, alphanumeric displays, electroluminescent devices, thin film tr ansistors, photovoltaic devices, and other vertically integrated electronic devices. Fi nally we have used b ack-side contacts method to construct a set of indepe ndently addressable pixels in a numeric display device application.


77 CHAPTER 4 LINE PATTERNING OF METALLIC ELECTRODES FOR LATERAL ECDS One of the greatest challenges in patterning of electronic devices is the complexity of the process to obtain finely structured electrodes. Conventional lithographic techniques described in Chapter 1 are currently used to pattern inorganic semiconductors for mass production. These techniques usually require multiple processing steps, tedious etching with plasma or solvents, and ultra-clea n processing environments. Several soft lithographic and direct pr inting methods such as microcontact printing101 and screen printing110 have been developed as alternative ap proaches which offer low cost and high resolution values. Line patterning, orig inally described by Hohnholz and MacDiarmid108, 124, 125 to pattern conducting polymers, is an ex cellent method to build fine structured electrodes on surfaces such as plastic or pape r. This method involves printing of patterns on a substrate using a commercial printer, followed by coating of the non-printed areas by a conductive, transparent PEDOT/PSS laye r or an electrole ss deposition of a conductor, such as gold. Subsequently, the printer ink is removed. The method benefits from the difference in reaction of the coati ng material to the substrate and the printed lines on it and allows selectiv e coating of substrates under normal atmospheric conditions as opposed to the other complex patterning techniques. In collaboration with the MacDiarmid Group at the Univ ersity of Pennsylvania, we have applied the line patterning method to build laterally configured polymer and metallic electrodes. The use of line patterning for polymer (PEDOT/PSS) electrodes and devices will be explained


78 later in Chapter 5. This chapter mainly describes the preparation of metallic electrodes using electroless metal depositi on and their uses in a vari ety of ECD applications. Electroless methods have attracted attenti on due to their metal deposition simplicity without electrical current. They involve the reduction of meta l ions onto a substrate by a chemical reducing agent and allow elaborate control on the metal de position. The process is either via immersion (galvanic displacement) where the deposition is limited by the exchange reaction between the metals, or autocatalytic where the deposition continues indefinitely as long as ther e are enough metal ions in the solution to be reduced.126-128 Immersion is terminated when the reducing metal completely covers the substrate surface yielding low final thickness values. In autocatalytic processes, the desired metal can be continuously deposited by a reducer in the solution. The electr oless autocatalytic deposition of gold onto plasti c substrates from a plati ng bath containing sodium Lascorbate as the reducing agent and sodium gold(I) thiosulfate as the gold complex114 is demonstrated to pattern gol d electrodes for ECDs. The process includes substrate activation steps of Sn, Pd, and Ni deposition prior to the gold depos ition. Preparation of Patterned Electrodes Patterns were generated using a computer aided design (CAD) software such as Adobe Illustrator and Microsof t Paint as shown in Figure 41a. These designs were then printed onto a plain transpar ency paper (substrate) as “negative” images using a commercial B&W LaserJet printe r. A cellophane tape was pu t onto the backside of the substrate to prevent metallization of the backside. Electroless de position of gold onto non-printed areas was carried out using a procedure given in the patent literature.109 Substrates were first placed in a slightly acidic SnCl2 solution which resulted in selective adsorption of Sn onto non-printe d regions due to the hydrophobic nature of the printer


79 ink. These activated regions were then subs equently exposed to a slightly acidic PdCl2 bath and a Ni bath to replace Sn with Pd and Ni, respectively. After homogenous coverage of Ni, the substrates were transf erred into toluene and the printer ink was removed by sonication in toluene to produ ce the patterned electrode. Finally, gold deposition was achieved by electroless reduc tion of gold onto Ni. Gold thickness was controlled by deposition time. 20 minutes of gold deposition was sufficient to generate highly conducting (~5 /square) and highly refl ective (%R > 80% at >600 nm) electrode surfaces. A more detailed procedure is given in Chapter 2. 500 m 500 m Transparency film Printer ink Electrolessgold deposition and removal of ink 300 m 300 m 300 m (a) (b) (c) Figure 4-1. Preparation of line patterned, gold electrodes. (a ) Computer generated designs (negative patterns) (b) Phot ographs of an interdigita ted electrode (IDE) and a 3x3 pixels pattern. (c) Reflective opti cal micrographs of the electrodes showing the patterns. White/Gray ar eas represent the uncovered regions Figure 4-1b shows photographs of the fi nal forms of the line patterned gold electrodes. For the interdigitated electrode (IDE) shown on the top, gold lines are 0.3 mm wide with an area of ~ 0.2 cm2. A 3x3 pixels mini displa y electrode with individual square areas of 0.01 cm2 is also shown to demonstrate the sizeable nature of the method. Surface resistivity between patterned lines/pixels was greater than 20 M /square (out of measurement range). Optical micrographs of the patterned electrodes in different


80 magnifications (Figure 4-1c) pr oved the selective deposition of gold on regions where ink is not present. The lateral resolution limit of the metallization on th e transparency film was determined to be ~30 m by 100x magnification of one of the patterned lines as shown in Figure 4-2. This indicates that th e insulating gap between the gold patterned lines can be as narrow as 60 m to prevent any shorts, which is mainly determined by the printer resolution and the ink removal accuracy. 30 m 30 m 30 m Figure 4-2. Optical micrograph of a 100x magnified line pattern ed gold substrate to show the resolution limit is down to 30 m. Black regions: Gold. Lateral ECDs Using Interdigitated Electrodes (IDEs) Lateral patterning of electrodes allows el ectrochemical deposition of two or more polymers on the same surface. An example of lateral patterning to create high contrast surfaces was shown in Chapter 3 where two different colored (red and blue) polymers were independently deposited and switched on a 2x2 pixels gold el ectrode. A typical display device with feature sizes on the order of 25-50 microns will cause the human eye to “color average.” Other applications, such as signs allow for larger feature sizes on the order of millimeters to centimeters. In this section, construction of lateral type ECDs comprising complementary colored polymers is presented. Each electrochromic layer can be addressed independently and requires elec trical insulation between lines or pixels. Lateral ECDs, in essence ECDs that operate on a single surface, can be constructed based upon the color mixing of two polymers deposited on a patterned surface. As an example,


81 a cathodically coloring and a complement ary anodically coloring polymer can be electrochemically deposited separately on th e patterned lines of an IDE as shown in Figure 4-3a. IDEs are ideal for lateral patter ning of electrochemical devices since they minimize the voltage drop problem by reduci ng the distance between the anode and the cathode. Due to the electrical insulation between adjacent fingers, deposition of the first polymer results in polymer films on altern ating fingers. Second polymer can then be deposited onto the remaining empty finge rs by flipping the IDE upside down and inserting it into an electrolyte solution containing the sec ond monomer. Finally, an ECD can be constructed by covering the active area by an ionically conducting media (i.e. gel electrolyte, ionic liqui d, or solid electrolyte) to allo w ion transport and a transparent plastic to protect the polymer layers. Using this simple con cept of “color averaging” and the high resolution output of the line patterning method, the resulting ECD (Figure 4-3b) can be observed to switch between two “match ed” color states. By changing the size and shape of these electrodes, the switching char acteristics and patterns of the ECDs can be manipulated. +-+IDE Deposition of Polymer 1 Deposition of Polymer 2With gel electrolyte In electrolyte solution EC SwitchingDevice assembly by applying gel electrolyte on top of the polymer films (a) (b) Figure 4-3. “Color averaging” in lateral type ECDs. (a) Electrochemical deposition of polymer films (b) EC switching of the resulting ECD.


82 PEDOT-PBEDOT-Cz Lateral ECDs Laterally configured dual polymer ECDs have subsequently been constructed utilizing the line patterned IDEs. It is importa nt to select the best complementary colored polymer pair to demonstrate the mixing of colors in the resulting ECDs. PEDOT, which turns from blue to transmissive sky blue upon oxidation, is employed as the cathodically coloring material due to its high stability, av ailability of its monomer, and the ease of electrosynthesis. PB EDOT-Cz, which turns from transmissive yellow to blue upon oxidation, is one of the few anodically colo ring polymers to complement PEDOTÂ’s color states. An absorption/transmission type ECD employing electrochromic layers of PEDOT and PBEDOT-Cz has been previously reported by our group which shows the eletrochromic and electrochemical compa tibility of this complementary pair.129 Figure 44a shows the lateral arrange ment of these two polymers along with their actual photographs on gold coated glass slides. PEDOT and PBEDOT-Cz are separately elec tropolymerized on alternating lines of a gold coated IDE (3 lines for each polymer, line area ~0.2 cm2) from their monomer containing electrolyte soluti ons (10 mM monomer in 0. 1M TBAP/ACN). PEDOT is potentiostatically deposited at +1.2V vs. Ag0. Accumulative deposition of PBEDOT-Cz on the empty lines is achieved by multisweep cyclic voltammetry from -0.2V to 0.9V vs. Ag0 as shown in Figure 4-4b. The deposition of the polymer on gold can be readily identified by the increase in current between 0.3V and 0.7V. It is important to note here that for a balanced switching, redox charges of the polymers should be matched. This is achieved by careful determination of the fina l deposition charges since the redox charge of a polymer is directly proportional to the de position charge. In order to obtain an equal number of redox sites, final deposition charge s of 24 mC and 9 mC are used for PEDOT


83 and PBEDOT-Cz, respectively. Figure 4-4c s hows the chronocoulometric data of these polymers which both yield 1 mC of redox charge. PBEDOT-Cz PEDOT Neutral Oxidized Neutral Oxidized - -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 First Scan I (mA/cm2)E (V) vs. Ag0 (a) (b) 0102030405060708090100-2 -1 0 1 2 Time (sec)9mC PBEDOT-Cz-1 0 1 -1 0 1 2 3 24mC PEDOTQ (mC)-1 0 1 E (V) vs. Ag0 (c) (d) Figure 4-4. (a) Arrangement of polymers for lateral type ECDs shown with their photographs on gold slides. (b) Poten tiodynamic deposition of PBEDOT-Cz on alternating fingers of the IDE. Sc an rate: 40 mV/s (c) Chronocoulometric matching of polymers. (d) EC switc hing of two complementary polymers (PEDOT and PBEDOT-Cz) on an interd igitated, line patterned electrode. Left: Bleached, reflective state. Right: Colored, absorptive state. Ionic medium: Gel electrolyte. After electrodeposition, the EC polymer ac tive layer was coated with an ionically conductive gel, followed by encapsulation. By applying opposite bias voltages to each polymer, a high contrast, laterally configured ECD is achieved. Figure 4-4d shows the EC -1.2V PEDOT PBEDOT-Cz +1.2V -1.2V PEDOT PBEDOT-Cz +1.2V

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84 switching of this device between -1.2V a nd 1.2V. At -1.2V (left) PBEDOT-Cz is transmissive yellow in its neutral form, while PEDOT is oxidized and highly transmissive. Switching the bias on the de vice (+1.2V, right) pr ovides matching blue colors. As such, the surface can be switched fr om a light yellow (gold) to a deep blue. The color and luminance match/contrast be tween two polymers can be controlled by adjusting the thickness and the redox states of the polymers. The ultimate switching characteristics of the lateral device can be tuned through further arch itectural engineering of the metallized pattern. Figure 4-5 shows the electrochemical switching characteristics of this device as it is switched between light yellow and blue. A vo ltage sweep of this device (Figure 4-5a) between -0.5V and +1.2V shows the oxidati on/reduction peaks of PBEDOT-Cz at scan rates of 40 mV/s and 100 mV/s as this polymer is selected to be th e working electrode. Two distinct reversible redox couples peaked at around 0.1V and 0.6V set the operational voltage window for this device. - -0.05 0.00 0.05 0.10 0.15 100 mV/s 40 mV/sI (mA)E (V) 05101520253035 -1.5 -1.0 -0.5 0.0 0.5 1.0 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Q (mC)Time (sec)I (mA)-1 0 1 E (V)-1 0 1 (a) (b) Figure 4-5. Electrochemical switching of the lateral PEDOT/PBEDOT-Cz device with PBEDOT-Cz being the working electrode . (a) Cyclic voltammogram of the device between -0.5V and +1.2V at scan rates of 40 mV/s and 100 mV/s. (b) Chronoamperometry and chronocolulometry of the device as the voltage is stepped between -1.2V and +1.2V.

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85 It is envisioned that the practical “two-sta te” operation of this device will require a standard battery with a fixed output voltage ca. 1.2V. In order to determine the electrical requirements, chronoamperometry and chronocoulometry are employed while the applied voltage is stepped between -1.2V and +1.2V with a delay time of 5 s at each voltage as shown in Figure 4-5b. As the devi ce voltage is stepped to +1.2V, the current reaches a maximum value of ~0.8 mA (1.3 mA/cm2) and stabilizes at ~0.1 mA (background current). Integration of this curren t as a function of time yields ~0.7 mC (1.2 mC/cm2). 95% of the total current decrease ta kes place in ~3 s which determines the switching time of this device. When compared to the switching times of < 1 s obtained from sandwich type devices constructe d from PEDOT and PBEDOT-Cz electrodes facing each other,130 the higher switching time of this de vice is attributed to the lateral configuration of the electr odes. The effect of the el ectrode line spacing on the performance of the IDE based lateral ECDs is presented in the next section. Lateral ECDs with Varying IDE Spacing Three IDEs with different active lane widths have been line patterned via electroless gold deposition in order to es tablish the dependence of switching time as a function of anode-cathode distance for late ral ECDs. Figure 4-6 shows the negative images of these IDEs where the white ar eas represent the conducting (gold coated) regions and black lines are insulating gaps to separate electrode fingers. All three IDEs are sized 7 x 50 mm with interdigitated lane lengths of 26 mm. The width of the insulating gaps is constant (a = 0.25 mm, black lines in Figure 4-6) and the widths of the lanes (x in Figure 4-6) are 3.38 mm, 1.56 mm, and 0.96 mm fo r 2-lane, 4-lane, and 6-lane IDEs, respectively.

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86 a 7 mm 26 mm x x Figure 4-6. Negative computer images of IDEs with varying finger widths. PProDOT-(Me)2 and PBEDOT-Cz are used as the complementary colored polymer pair and they are electrochemically polym erized onto alternating IDE fingers by multisweep cyclic voltammetry as shown in Figure 4-7 for a 2-lane IDE. Redox switching charges of these polymers are matched by c ontrolling the polymer deposition charge as explained in the previous section. Afte r electrodeposition, both of the polymers are individually switche d in a 0.1M TBAPF6/PC electrolyte solution for conditioning. -1.0- -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 J (mA/cm2)E (V) vs. Fc/Fc+ -0.8-0.6-0.4- -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 J (mA/cm2)E (V) vs. Fc/Fc+ (a) (b) Figure 4-7. Multi-sweep CV electro polymerization of (a) PProDOT-(Me)2 and (b) PBEDOT-Cz from their monomer electroly te solutions onto a 2-lane IDE.

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87 Prior to device assembly, PProDOT-(Me)2 is reduced and PBEDOT-Cz is oxidized and both polymers are in their absorptive blue states. The lateral ECD assembly is carried out by placing the IDEs on a plastic support facing up and co ating the active layer with an ionically conductive gel as shown prev iously in Figure 4-3. Figure 4-8 shows potentiodynamic sweeps of the PProDOT-(Me)2 (working electrode) lane s of the all three ECDs between -1.0 V and +0.8V. It is evid ent from the separation of the oxidation and reduction peaks that the 2-lane device operation (black curve) is considerably slower as the redox switching of the polymer lags behind the scan rate (50 mV/s). -1.2-1.0-0.8-0.6-0.4- -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 J (mA/cm2)E (V) Figure 4-8. Voltage sweep of 2lane (black), 4-lane (red), and 6-lane (green) lateral ECDs comprising PProDOT-(Me)2 (working electrode) a nd PBEDOT-Cz (counter electrode) as the complementary colore d polymer pair. Scan rate: 50 mV/s. Reflectance characterization is carried out by illuminating the devices with a beam size larger than the width of the IDEs (> 7 mm) and monitoring the reflected light at 611 nm where the complementary polymer pair has the highest optical contrast. As such, the measured reflected light is the average gold reflectance m odulated by the polymers. In order to study the EC switching kinetics of the devices with different lane widths, a large magnitude potential step from -1.0V to +0.8 V is applied at t=0 and the %R change is monitored as a function of time as shown in Figure 4-9a. The switching times to reach

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88 85% of the full contrast are 4.3 s, 1.5 s, and 0.8 s for the 2-lane, 4-lane, and 6-lane devices, respectively. Figure 4-9b shows the dependence of this switching time as a function of the average anode-c athode distance (x+a, see Figu re 4-6) in interdigitated lateral ECDs. The first data point is taken fr om a porous type ECD described in Chapter 3 to set an experimental lower limit for the switching time (t = 0.2 s) where the anodecathode distance is 50 m. The extent of interdig itation noticeably improves the switching performance of latera l ECDs due to smaller diffu sion distance of doping ions and minimal electrolyte resistance. However, the increase in the number of insulating gaps results in loss of active area and may limit the space efficiency of a device (e.g. when a 2-lane IDE is replaced with a 6-lane IDE, the active area decreases by 13%). Further optimization on the gain of kinetic performance against the loss of active area depends on the device application. 01234567 70 75 80 85 90 %R at 611 nmTime (Sec) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Switching Time (Sec)Distance, x+a (mm) x = line width a = gap width (0.25 mm) (a) (b) Figure 4-9. (a) The %R changes of the 2-lane (black curve), 4-lane (red curve), and 6lane (green curve) devices as a function of time as they are switched from -1.0V to +0.8V. (b) Switching time to reach the 85% of the full contrast as a function of the distance be tween the anode and the cathode for lateral ECDs.

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89 Finally, Figure 4-10 shows photographs of the EC switching of a 4-lane device between its colored (-1.0V) and reflective (+0.8V ) states. The color cha nge initiates at the insulating gaps and moves laterally across the lanes. -1.0V +0.8V Figure 4-10. EC switching between an absorptive blue state (-1.0V, le ft) and a reflective state (+0.8V, right). Other Applications of Line Patterning Lateral ECDs using line patterned electro des usually use at least two different (complementary) polymers to obtain color co ntrast or color match. As an alternative approach, a reflective type lateral ECD is devised comprising different polymer thicknesses of PEDOT on the anode and cathode. Specifically , a 9-pixel line patterned gold substrate (pixel area ~ 0.25 cm2) is used to show a “cross pattern” by shorting four of the corner pixels with the center pi xels as shown in Figure 4-11. PEDOT is electrochemically polymerized on these five (c ross) pixels and the re maining four (side) pixels separately with a total deposition char ge of 16 mC and 33 mC , respectively. Prior to device operation, PEDOT on the cross pixels is electrochemically reduced to deep blue color and PEDOT on the side pixels is fully oxi dized to show the gold layer beneath. In a two-electrode configuration in a liquid electrolyte (0.1M TBAPF6/PC), cross pixels are assigned to be the working el ectrode and the side pixels are grounded. As shown on the

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90 left of Figure 4-11, the blue cr oss is visible at -1.0V. When the voltage is stepped to 0.2V, PEDOT on the cross pixels is completely oxidized with partial reduction of the thicker PEDOT on the side pixels. This resu lts in a no-contrast gold surface at -0.2V through complete bleaching of the cross pixels without coloration of the side pixels. -0.2 V -1.0 V Figure 4-11. EC switching of a cr oss patterned PEDOT device to yield high contrast (left, -1.0V) and no contrast (right, -0.2V) surfaces. In another application, electrochemical deposition and electroc hromic switching of PEDOT films on line patterned ITO/plastic substrates is presented. The substrates, prepared by the MacDiarmid Group, comprise ITO coated, interdigit ated conducting lines (line width: 4 mm) parallel to each other and they are separated by non-conducting gaps (gap width: 1 mm). The electrode prepara tion includes printing of the negative pattern onto an ITO coated plastic, removal of the ITO from the non-printed (unprotected) regions, and removal of the printer ink by soni cation in toluene to yield interdigitated ITO patterns. PEDOT films were potentiost atically grown (E = 1.00V vs. Ag/Ag+) on substrates from 0.01M EDOT in 0.1M LiClO4/PC. Film deposition was homogenous on alternating lines without a th ickness gradient down the el ectrode with a few defects possibly due to the quality of ITO on plasti c. Electrochromic switching of PEDOT in a liquid electrolyte betwee n +1.1V (clear) and -1.1V (blue) vs. Ag/Ag+ is shown in Figure 4-12.

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91 Figure 4-12. EC switching of PEDOT on line patterned, interdigitated ITO/Plastic electrodes. Oxidation (+1.1V) Reduction( 1.1V)

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92 CHAPTER 5 ALL ORGANIC ECDS ECDs utilizing conjugated polymers as electroactive layers have received increased attention due to their ease of color tuni ng properties, fast switching times, and high contrast ratios. Our group ha s reported polymer based ECDs,77, 92, 130, 131 including a transmissive/absorptive type complement ary colored polymer ECD with an overall colorimetrically-determined luminance change of 55% in the visible region, which can be switched more than 20,000 times between its colored and transmissive states.77 Throughout the world, a number of groups have utilized EC polymers as at least one component of an ECD.34, 49, 70, 81, 112, 132, 133 Traditionally, ITO on either glass or plastic has been used as the electrode material in ECDs and electrochromic polymers were deposited electrochemically or cast from solution. While previous workers have claimed all-polymer ECDs,49, 70, 72, 134, 135 their devices comprised ITO as the electrode mate rial as no suitable highly conducting and transmissive organic polymer was available. Using ITO coated plastic substrates, many complementary colored polymers have been inve stigated to obtain flexible and polymer based ECDs . DePaoli et al. have used polypyrrole and polythiophene derivatives as complementary polymer pairs in flexible ECDs using ITO coated PET substrates as the electrode material.71 Similarly, Gazotti et al. blended conducting polymers with a solid polymer electrolyte and construc ted ECDs on ITO coated plastics.72 Here, we report the construction and characterization of a truly all-polymer ECD by replacing ITO with a conducting polymer, namely poly(3,4-ethyle nedioxythiopene)-poly(styrene sulfonate)

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93 (PEDOT/PSS).79 PEDOT/PSS (chemical structure show n in Figure 5-1) is a stabilized aqueous dispersion which is commercially pr oduced in large quant ities by Bayer A.G. (Baytron-P) and Agfa-Gevaert. Since its discovery in the late 80s,136, 137 PEDOT has proven to be an outstanding polymer for its electrochromic properties, high conductivity, and high stability in the doped form.19 It has already found useful applications as antistatic film coatings,138, 139 electrochromic windows,140 and as a hole injection material in polymer OLEDs and PLEDs.141 The layer-by-layer electr ostatic adsorption of a sulfonated derivative of PEDOT has been investigated by our group where the multilayer thin films exhibit a fast and revers ible redox switching be havior in aqueous media.75, 76 As another application, research ers from Linköping University and Acreo have combined an electrochemical transistor wi th an ECD to build an active matrix paper display using thin films of PEDOT/PSS.78 S O O x m+ SO3 -SO3 -M+mn Figure 5-1. Chemical st ructure of PEDOT/PSS. In the following two sections fabricati on and characterizatio n of ECDs using PEDOT/PSS on patterned and non-patterned el ectrodes are presented. Patterning is achieved using the line patterning method previously described in Chapter 4. PEDOT/PSS is processed from its aqueous dispersion either by ro ll-coating or spin coating on flexible, transparen t electrodes. EC polymer film s are then electropolymerized on these polymer electrodes.

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94 Line Patterned PEDOT/PSS Electrodes In this work, electrochemical depos ition and electrochromical switching characteristics of PEDOT and PBEDOT -Cz on conducting PEDOT/PSS (Baytron P) coated transparency films are discus sed. Line patterned PEDOT/PSS on common transparency films are used as substrates which allow region-specific deposition of cathodically coloring electrochromic PE DOT (EC PEDOT) and anodically coloring PBEDOT-Cz. Patterning was achieved using the line patterning method originally introduced by Hohnholz and MacDiarmid.108 Interdigitated lines were printed on plastic transparency film substrates using a comm ercial B&W laser prin ter and PEDOT/PSS was smear coated on these substrates. Due to hydrophobicity of the pr inter ink, PEDOT/PSS selectively deposits on the more polar non-printed lines as sh own in Figure 5-2. The color shown on the non-printed regions is the actual color of Bayt ron-P coated films measured by a Minolta CS-100 colorimeter. After drying and removal of toner ink using toluene, interdigitated lines of PEDOT /PSS were obtained. A copper ta pe was attached to the top of the films to provide electrical contact. Three different types PEDOT/PSS coated transparency films were used: One-time coated (S1) Two-times coated (S2) Three-times coated (S3) The surface resistance values of the PEDOT /PSS coated substrates were measured as shown in Figure 5-2. Rs is the resistance between the two ends of the printed film and s is the surface resistivity of an electrode pad in units, k / . Results are given in Table 5-1. Since the coating is carried by smeari ng the PEDOT/PSS on the transparency film by a test tube, these values only give a rough idea about th e increase of conductivity by

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95 increasing the number of coats. The resu lts suggest that th e second coating (S2) has a greater effect on increasing the conductivity relative to the third. Rs Rs Figure 5-2. Schematic representation of a PEDOT/PSS (Baytron P) coated, interdigitated plastic electrode. Table 5-1. Surface resistance (Rs) and surface resistivity ( s) values of PEDOT/PSS coated films. S1 S2 S3 Rs (k ) 52 27.1 24.4 s (k / ) 40.9 15.4 10.8 PEDOT Deposition EC PEDOT films were potentiostatica lly deposited (E = 1.10V vs. Ag/Ag+) on substrates (S1, S2, S3) from 0.01M EDOT in 0.1M LiClO4/PC. Monomer oxidation occurred at 0.83V vs. Ag/Ag+. Charge densities of films on S1, S2, and S3 were 40 mC/cm2, 45 mC/cm2 and 45 mC/cm2, respectively. A stainless st eel plate was used as the counter electrode. Films we re cleaned with monomer free electrolyte solution upon deposition. EC PEDOT deposited films were switched in el ectrolyte by a bipotentiostat between –1.1V and 1.1V vs. Ag/Ag+. The percent transmittance (%T) of the line patterned PEDOT/PSS coated films taken through the interdigitated electrodes (IDE s) is on the order of %80 in the visible

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96 region vs. air as shown in Figure 5-3. EC PEDOT deposited more on the top of the electrode (closer to the contact) rather than the bottom. Th is is due to the “iR” drop down the interdigitated lines due to the hi gh resistance of the PEDOT/PSS (~10 k ) compared to the resistance of line patterned gol d lines described in Chapter 4 (~20 ). As such, the potential drops along the electrode surface, which gives a thickness gradient of EC PEDOT down the electrode. Th ere are also deposition def ects throughout the electrode, which are possibly because of non-conduc ting sites on the electrode lanes. 300400500600700800 0 20 40 60 80 100 %TWavelength (nm) S1 S2 S3 Figure 5-3. %Transmittance of PEDO T/PSS coated substrates vs. air. Optical micrographs of th e oxidized PEDOT films are shown in Figure 5-4. The first picture (a) shows the resolu tion of the printed lines and it seems that there is residual PEDOT/PSS deposited between the patterned line s. It is possible that the PEDOT/PSS has penetrated the laser printer ink. The second picture (b) show s the top of the EC PEDOT at the meniscus which developed during electrodeposition with a printed line crossing the interface. This pict ure was taken from the solution-air meniscus of the film.

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97 A closer look at the printed line at the interface (c) shows that PEDOT/PSS partially deposits on the insulating line (red circle). Th e EC PEDOT deposits on these sections and bridges the conducting lines creating a short circuit between the layers. (a) (b)(c) Figure 5-4. Optical microsc ope pictures of EC PEDOT film on PEDOT/PSS. (a) EC PEDOT film deposited between micr o-printed lines (b) EC PEDOT – PEDOT/PSS interface at the meniscus (c) Magnification of the interface to show the short between th e PEDOT lines (red circle) Despite the defects on the patterned line s, EC PEDOT could be made to deposit only on the interdigitated lines desired. Figure 5-5 shows EC switching of PEDOT between its colored (left) a nd bleached (right) states. Th e EC PEDOT deposits better closer to the meniscus than it does on the bottom (iR drop) . The EC PEDOT changes its color state from blue to transparent upon oxi dative doping and this process is reversible

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98 between 25-100 switches. There is no colo r change evid ent in the line patterned PEDOT/PSS suggesting that it does not switch its redox state after it has been dried. +1.1V -1.1V +1.1V -1.1V Figure 5-5. Redox switching of EC PEDOT be tween (–1.1V) and (+1.1V) vs. Ag/Ag+ Another batch of Baytron P coated, line patterned interdigitated electrodes on PET substrates with improved PEDOT/PSS deposition was used to investigate the electrochemical deposition and electrochr omic switching of PEDOT and PBEDOT-Cz. Resistivity (RS) values of these films varied subs tantially in the range of 20-180 k . Electrochemical deposition of EC PE DOT films on these improved PEDOT/PSS substrates was carried out by following th e procedure given earlier to yield a homogenous film with a minimal IR drop as shown in Figure 5-6a. EC PEDOT deposits on every other line as a result of the line pa tterning and switches be tween its redox states reversibly at +1.2V and -1.2V vs. Ag/Ag+. An optical micrograph of the film (Figure 56b) proved that there are no shorts between th e lines as the line patterning method was more fully developed. The middle line (b lue colored) shows th e EC PEDOT deposited line. The black spots on the micrograph are due to residual toner inks from the laser pointer. They do not result from electrodepos ition because they also exist on the nondeposited and insulated sites. Chrono-coulometry was performed by st epping the potential between –1.2V (25 sec) and +1.2V (25 sec) vs. Ag/Ag+ to ensure that the con centration of the redox active species at the electrode is zero (Cottrell behavior). Dopi ng/dedoping charge was found to

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99 be ~ 1.5 mC/cm2 with a maximum doping current ~ 0.12 mA/cm2(imax). The first 25 square-waves resulted in no decrease in imax. After 200 square-waves, imax dropped to 0.08 mA/cm2 which indicates a 30% loss in electroactivity of the film. Oxidation (1.2V) Reduction (-1.2V) Oxidation (1.2V) Reduction (-1.2V) (a) (b) Figure 5-6. EC PEDOT deposited electrode. a-)Electrochromic switching of the PEDOT between its redox states. PE DOT deposited area ~3 cm2, deposition charge ~ 14 mC/cm2 b-) Optical micrograph of PEDO T deposited (middle line) and non-deposited lines. PBEDOT-Cz Deposition PBEDOT-Cz, a multiply colored, anodical ly coloring electrochromic polymer, is useful as a complementary polymer for cathod ically coloring EC PEDOT. It switches between pale yellow (reduced) and blue (fully oxidized) with an intermediate green color. EC PBEDOT-Cz films were potentiostati cally deposited (E = 0.5V vs. Ag/Ag+) on substrates from BEDOT-Cz (0.0025 M) in TBAP (0.1M) /ACN and they were switched in a monomer free electrolyte betw een –1.2 V and 0.7V vs. Ag/Ag+. Figure 5-7a illustrates the color change of a PBEDOT -Cz film between its redox st ates in a three-electrode electrochemical cell. Quantitative measur ement of color was carried out using a colorimeter. Figure 5-7b shows the color ch ange of PBEDOT-Cz on Baytron P coated electrode at various potentia ls based on the L*a*b values recorded by the colorimeter. Since the measurement area of the colorimeter (b lack circle in Figure 5-7b) is larger than the targeted area on the subs trate (PBEDOT-Cz deposited line), the color measured by the colorimeter does not exactly match with the color in real picture. Mixture of colors

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100 with neighboring PEDOT/PSS lines results in contribution of “sky blue” to the measured color. -1.2 V0.4 V 0.7 V -1.2 V0.4 V 0.7 V (a) (b) Figure 5-7. Electrochromic sw itching of PBEDOT-Cz in TBAP (0.1M) /ACN electrolyte solution. Actual colors of PBEDOT-C z on Baytron P coated substrate based on L*a*b values measured by Minolta CS-100 colorimeter. Throughout the experiments, low conductivity of the Baytron P IDEs compared to the line patterned gold IDEs has been a major drawback for both EC film deposition and switching. The surface resistance of some of th e electrodes increased with exposure to the laboratory atmosphere and it became impo ssible to deposit any polymer films. The degeneration of electrodes is possi bly due to humidity or oxidation in air. In an attempt to increase the electrode conductivity, electrochemical reducti on of a thin layer of gold on the substrates was performed. This was at tempted using potentiostatic methods (E = 0.9V vs. Ag/AgCl) from an aqueous solution of Au2SO3. However, either Baytron P dissolved in aqueous gold solution or resistiv ity was too high to electrochemically deposit gold on the substrates (Depositi on current was as low as 1 A/cm2, no gold deposition was observed). After gold deposition, these el ectrodes lost their c onductivity irre versibly (Rs ~1200 K ) and never recovered even t hough desiccated under vacuum. Highly Conducting PEDOT/PSS Electrodes In this section, the use of PEDOT/PSS complex as the electrode material for polymer based ECDs is described in order to form a device that is fully constructed from +0.7V -1.2V

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101 organic and polymeric components. We use the PEDOT/HAPSS aqueous dispersion (Agfa-Gevaert) as the resulting films are highly transmissive in the visible region, have high conductivity, and are unreactive (do not dedope) under th e electrochemical conditions employed. Importantly, when used as the electrode material, PEDOT-HAPSS films do not return to the non-conducting form in the ECD’s operating voltage range. In order to evaluate the suitabil ity of PEDOT-HAPSS films as electrode materials, the films were first subjected to a reducti ve potential (–1.5V vs. Fc/Fc+) for 3 minutes in 0.1 M TBAPF6/acetonitrile. No significant change in electrode conductivity or transparency was observed. Secondly, the current-potential ch aracteristics were obtai ned by CV scanning of the films between -1.5V and +1.0V (vs. Fc/Fc+). Very low current values (<20 A/cm2) were obtained relative to t hose that we observe for sw itching the EC polymers (~3 mA/cm2), ensuring that the PEDOT-HAPSS elec trodes are not redox active in this potential window. Once dried, they are well adhered to the plastic substrate and are insoluble in water and the electrolyte solutio ns used for electrochemical deposition and switching of EC polymers. Using PEDOT-HAPSS as the electrode material brings out the advantage of making flexible, st able, and truly all-organic ECDs. The conductivity of the PEDOT-HAPSS fi lms was determined both from spin coating of PEDOT-HAPSS on glass slides ( 44 nm) and casting free-standing films (18 m). Using 5% N-methylpyrrolidone (NMP) or 5% diethylene glycol (DEG) in the film processing solutions, the conductivity increase d from 0.6 S/cm to 120 S/cm independent of processing method. Diffe rent types of PEDOT/PSS both from Bayer and Agfa were cast and the same conductivity enhancemen t was observed upon mixing with high boiling point solvents as shown in Table 5-2. The specific mechanism for conductivity

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102 enhancement is not well delineated, but likel y has to do with changes in the morphology of the p-doped conjugated polymer: polyelectro lyte simplex. Conductivity enhancements of a lower magnitude have been observed by Ghosh et al.142 in blends of PEDOT/PSS in polar carrier polymers that have bivalent meta l ion additives. They have also reported a similar conductivity increas e by adding polyols such as glycerol or sorbitols.143 Table 5-2. Conductivity enhancement of PE DOT/PSS using high boiling point additives. BAYTRON (V4080) PEDOT/PSS (S/cm) AGFA (CH02663) PEDOT/PSS (S/cm) AGFA PEDOT-HAPSS (S/cm) No additives 6.2 x 10-4 6.8 x 10-2 6.3 x 10-1 With 5% NMP 7.1 x 10-4 80.5 120 With DEG+MeOH N/A 65 115 EC Polymers on PEDOT-HAPSS Electrodes In this study, electrodes we re prepared by spin coating of aqueous PEDOT-HAPSS (mixed with one of the additives mentioned above) on a commercial plastic transparency film. Multiple layers of PEDOT-HAPSS were achieved by hot air drying of the films between coatings and subsequent air oven dr ying of the multi-layer film. After three coatings, the surface resistivity of the electrodes decreases to 600 sq (300 nm layer thickness) while remaining highly transmissive throughout the visible region as shown in Figure 5-8. Table 5-3 lists th e surface resistivity change s of PEDOT-HAPSS coating as the number of layers increases. Even after three coatings, the transmittance is always 75% which is comparable to that of an ITO electrode as shown. While additional layers of PEDOT-HAPSS enhanced the electrode con ductivity, it reduced the transparency and the quality of the films. The decrease in the surf ace resistivity of the films is not linear with the increase in the numb er of layers and tends to saturate for thicker films.

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103 Table 5-3. Surface resistivity values of PEDOT-HAPSS # of Coatings NMP+PEDOT-HAPSS ( sq) MeOH+DEG+PEDOT-HAPSS ( sq) 1 8000 1800 2 1400 850 3 1100 600 400500600700800 0 10 20 30 40 50 60 70 80 90 100 ITOiii ii i% T Wavelength (nm) Figure 5-8. Percent transmittance (%T) of the PEDOT-HAPSS coated transparency film electrodes in the visible regi on with (i) one layer, (ii) two layers, and (iii) three layers. Electrodes with 3 layers yield a surface resistivity of 600 /sq with an average %T value of 77% through the vi sible spectrum. %T spectrum of a 200 nm ITO electrode (bold line) is also shown for comparison. In order to test the electrochemical and optical competency of PEDOT-HAPSS electrodes compared to the ITO electrodes, EC polymers were electrochemically synthesized directly onto these plastic film supported PEDOT-HAPSS electrodes from their monomer solutions. The polymerization was achieved either potentiostatically at potentials slightly above th e oxidation peaks of the mono mers or potentiodynamically by employing repeated cycles between a nega tive potential and th e monomerÂ’s oxidation potential. Figure 5-9a is an example of the potentiodynamic deposition of PEDOT on these PEDOT-HAPSS electrodes at 25mV/s from a 0.01 M of EDOT in 0.1M TBAClO4/PC. Monomer oxidation o ccurs at 1.2V vs. Ag0 and accumulative deposition of

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104 25 mC/cm2 PEDOT was achieved by cycling 20 times between potentials -0.5V and 1.4V. The redox (switching) charge of the polymers was obtained using chronocoulometry where the polymer is switche d from its neutral to oxidized state and the charge was monitored as a function of time. It is important to balance the charges of the complementary polymer films in electrochr omic devices since the charge required to oxidize one polymer should match the charge to reduce the other. In other words, redox charges of two complementary films should be matched in order to provide a balanced number of redox sites for sw itching. Figure 5-9b shows the redox charge of PEDOT as a function of the deposition (polymerization) charge. Therefore, the charge needed to switch a PEDOT film can be calculated from its polymerization charge. Once the films are made, they were sw itched in a monome r-free electrolyte solution. A cyclic voltammogram of a PE DOT film on PEDOT-HAPSS is shown in Figure 5-9c. The polymer gives a broad redox couple between -0.7V and +0.8V with an E1/2 value of ~ 0.1V vs. Ag0. In order to measure the optical contrast and electrochromic stability, PEDOT coated PEDOT-HAPSS elect rodes are studied us ing single-wavelength spectroscopy at a wavelength where the polymer film has its highest EC contrast (Figure 5-9d). The transmittance (%T) change is monitored as a function of time upon electrochromic switching between -0.8V (n eutral, colored) and +1.0V (oxidized, bleached). The switching time to reach the 95% of the full contrast was measured to be ~ 25 seconds and the initial contrast of 44% only dropped to 38% over 30 full cycles.

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105 - -0.010 -0.005 0.000 0.005 0.010 0.015 J (mA/cm2)E (V) vs. Ago01020304050 0 2 4 6 8 10 Qred(mC)Qdep(mC) (a) (b) -1.5-1.0- -0.02 -0.01 0.00 0.01 0.02 0.03 J (mA/cm2)E (V) vs. Ag00246102030405060 40 60 80 %T Time (minutes) (c) (d) Figure 5-9. a) Accumulative s ynthesis of PEDOT from its monomer solution at 25 mV/s. b) Redox (switching) charge (Qred) as a function of deposition (polymerization) charge (Qdep) for PEDOT. Solid line represents the linear fit of the data points. c) Cyclic voltammogram of a PEDOT film in a monomerfree electrolyte solution at 10 mV/s. d) Percent transmittance change of the PEDOT film recorded at 607 nm as a function of time as the potential is switched between -0.8V and +1.0V. Figure 5-10a, b, and c show photographs of EC polymer films of PEDOT, PBEDOT-B(OC12)2, and PBEDOT-NMeCz) on PEDOT-HA PSS electrodes, respectively. The oxidized states are shown on the left a nd the neutral states are shown on the right. The films are electropolymerized from thei r monomer solutions and switched in monomer-free electrolyte soluti ons. They are homogenous with very little defects and their EC switching is fully reversible. The success in EC polymer deposition and switching ensure that these PEDOT-HAPSS el ectrodes provide a suitable system to

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106 characterize EC polymers and devices. In the ne xt section, all-orga nic ECDs using these EC polymer films as complementary pa irs are presented. PEDOT, PProDOT-(Me)2, and PBEDOT-B(OC12)2 are used as cathodically colori ng polymers and PBEDOT-NMeCz is used as an anodically coloring polymer. (a) (b) (c) Figure 5-10. Photographs of EC polymers on PEDOT-HAPSS electr odes in colored and bleached states. Left: Oxidized, Righ t: Neutral. Electrode areas ~ 2 cm2. a) PEDOT, b) PBEDOT-B(OC12)2, and c) PBEDOT-NMeCz. All Organic Electrochromic Devices Two ECDs, using different complementary pairs of EC polymers, were assembled as shown by the schematic diagram in Figur e 5-11 and tested to demonstrate the operation of the all-polymer ECD. ECDs we re assembled by arranging two EC polymer films (one doped, the other neutral) facing each other separated by a polymer based gel electrolyte. Plastic Transparency Film PEDOT-PSS Anodically Coloring EC Polymer Cathodically Coloring EC Polymer Polymer Gel Electrolyte Plastic Transparency Film PEDOT-PSS Anodically Coloring EC Polymer Cathodically Coloring EC Polymer Polymer Gel Electrolyte Figure 5-11. Schematic representation of the transmissive/absorptive type ECD constructed from all-polymer components.

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107 Absorptive/transmissive ECDs The first device is an example of an electrochromic window, having distinct absorptive and transmissive states as recently studied by our group on ITO/glass electrodes.77 PProDOT-(Me)2 and PBEDOT-N-MeCz were us ed as the cathodically and anodically coloring polymers, resp ectively. Initially, PProDOT-(Me)2 is in its oxidized (sky blue) form and PBEDOT-N-MeCz is in its neutral (pale yellow) form; hence the device is observed as a relatively transmissi ve green. Application of a voltage (negative bias to PProDOT-(Me)2) switches the oxidation states of the polymers so that both polymers are colored. Figure 5-12a shows the spectroelectrochemistry of such a device as a function of applied voltage and demons trates a maximum transmittance change ( %T) of 51% at 540 nm. The ITO/glass based device comprising the same EC polymer pair has a transmittance change of 56%, proving the co mpatibility of the PEDOT-HAPSS as an electrode material in these ECDs. The sw itching time to reach 80% of the highest contrast is 8 seconds (<1 s econd for ITO device). The slower switching time is expected and attributed to the higher surface re sistance of the PEDOT-HAPSS electrodes compared to the ITO coated electrodes. The results of Figur e 5-12a also nicely demonstrate how the absorption of the device can be continuously tuned as a function of voltage, quite different from onoff type devices such as t hose based on scattering from polymer dispersed liquid crystals.

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108 400500600700800900 0 10 20 30 40 50 60 70 80 90 100 %T Wavelength (nm) +1.2 V +0.5 V +0.1 V -0.1 V -0.5 V -0.75 V -1.2 V-1.5-1.0- 0 10 20 30 40 50 60 70 % Relative LuminanceE (V) -1.5-1.0- 0 10 20 30 40 50 60 70 % Relative LuminanceE (V) (a) (b) 05000100001500020000250003000035000 20 30 40 50 60 70 80 020406080100120 20 40 60 80 %T (540 nm)t (seconds)% T (540 nm)# of switches (c) (d) Figure 5-12. Optical characterization of a complementary colored ECD using PProDOT(Me)2 as the low bandgap, cathodically coloring polymer and PBEDOT-NMeCz as the high bandgap anodically coloring polymer. a) Spectroelectrochemistry of the device obtained from UV-Vis-NIR spectrophotometry. At positive volta ges (PBEDOT-Cz: neutral, PProDOT(Me)2: oxidized), the peak at 420 nm is due to the * transition of PBEDOT-Cz. As the device is switched to negative voltages, this peak diminishes and a new absorption peak at 580 nm appears which is due to the * transition of PProDOT-(Me)2. %T at 540 nm was measured to be 51%. b) Voltage dependent percent relativ e luminance change of the device. Photographs are taken at two extreme st ates of the device, namely, colored and bleached. c) Repeated switching stability measured at 540 nm. Inset shows the switching of this device be tween its redox states in 20 second double potential steps. d) A photograph of th e ECD in its colored state, bent to show the flexibility. Figure 5-12b shows the voltage dependence of th e colorimetrically determined relative luminance change of the device under transmission of white light from a standard

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109 5000 K white light source measured by a Minol ta CS-100 colorimeter, along with photographs of the extreme states. In the ble ached state (transmissive green), the device possesses 67% relative luminance with a positive voltage bias to the PProDOT-(Me)2. The slight green color is due to the * absorption from PBEDOT-N-MeCz which has a max at 430 nm. Upon switching the voltage bias , the device becomes highly absorptive and this value decreases to 7% in the co lored state (absorptiv e blue). We employed single-wavelength spectrophotometry to inve stigate the long-term stability of the PProDOT-(Me)2/PBEDOT-N-MeCz device at = 540 nm while the voltage was repeatedly stepped between –1.2V and +1.2V with a 10 second delay at each state as shown in Figure 5-12c. The high stability of thes e devices is evident as the initial contrast of 45% T only decreased to 43% T (less than 5% contrast loss) after 32,000 switches over a period of 3.5 days. Finally, Figure 5-12d shows a photograph of this ECD (Active area = 2 cm2) held in its colored state, bent to demonstrate its flexibility. In addition to the relative luminance (%Y) measurements, colorimetry also allows us to quantify the color coordi nates of an ECD at different voltages. Using the CIE 1931 standards, a two-dimensional xy representation is utilized, known as the chromaticity diagram in which x and y values are obtained using the Minolta colorimeter. A point on the xy diagram gives the coordinate s (hue and saturation) of the color, leaving out the relative brightness of a material. Figure 513 shows the color change of the PProDOT(Me)2/PBEDOT-NMeCz device between -1.2V and +1.2V. Here, it is important to note that as the device follows the path from the blue region to the green region, the brightness of the device increases by 60% (Figure 5-12b).

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110 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 y x Figure 5-13. Representation of the color change of the PProDOT-(Me)2/PBEDOTNMeCz device on the CIE 1931 xy chromaticity diagram. One of the most important criteria methods to characterize an ECD is its coloration efficiency ( ) as explained previously in Chapters 1 & 2. The composite coloration efficiency27, developed in our group for practical measurement of the power efficiency of a device, is evaluated for a PProDOT-(Me)2/PBEDOT-NMeCz device. A nearly complete 95% of the full switch is r eached for this device in 13 seconds. Table 5-4 shows the coloration efficiency values of this devi ce recorded at various switching times when switched from the absorptive state (%Tc=29.0%) to the bleached state (%Tb=76.2%). As an example, when 95% of the to tal contrast is obtained, the of the device is 356 cm2/C. Table 5-4. Coloration Efficiency values of a PProDOT-(Me)2/PBEDOT-NMeCz device. See Chapter 1 for the explanation of the parameters. T # of full switch Switching time (s) %Tb Qd OD (cm2/C) 47.2 100 30 76.2 1.384 0.419 303 44.8 95 13 73.8 1.141 0.415 356 42.5 90 11 71.5 1.059 0.392 367 40.1 85 9 69.1 0.984 0.377 383 37.7 80 8 66.7 0.916 0.362 395

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111 Dual-colored ECDs Another set of electrochro mic polymers was selected for a second device to demonstrate EC switching between two absorpti ve color states (blue and red) with a transmissive intermediate state. Employing the same device scheme (Figure 5-11), the device was constructed from two cathodically coloring polymers, PEDOT (blue to sky blue) and PBEDOT-B(OC12)2 (red to sky blue). Figure 5-14a shows the spectroelectrochemical data of the PEDOT /PBEDOT-B(OC12)2 device at voltages varying between –1.5V and +1.5V. With a negative bias to PEDOT the polymer is in its neutral state with a * transition max at ~630 nm which is responsible for the blue color. At this bias voltage, PBEDOT-B(OC12)2 is oxidized and quite transmissive (see Figure 5-10b for the color states of this pol ymer). As the device bias voltage of PEDOT is switched to positive values, a new * absorption band appear s due to the PBEDOTB(OC12)2 with three peaks between 450-580 nm gi ving the device its red color state. Simultaneously, the PEDOT bleaches. The per cent relative luminan ce change of this device is shown in Figure 5-14b al ong with the photogra phs of three color states (blue at 1.5V, transmissive gray at 0.0V, and red at +1.5V). The relative lu minance of the device remained low in the two different absorptive states (~17% and 32% at -1.5V and +1.5V, respectively) with change of color from blue to red, indicating how these devices can be employed as bistable color de vices. However, at intermediate voltages, these two polymers are partially oxidized and transmissive yi elding high luminance values (such as 38% at E = 0.0V).

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112 400500600700800 0 10 20 30 40 50 60 70 80 %TWavelength (nm) -1.5 V -1 V -0.5 V +0.1 V +0.5 V +0.8 V +1 V +1.5 V (a) -1.5-1.0- 15 20 25 30 35 40 Relative Luminance (%Y)E (V) +1.5V 0.0V -1.5V (b) Figure 5-14. Optical characterization of a two-colored ECD using PEDOT and PBEDOTB(OC12)2 as the EC polymers. a) Spectro electrochemistry of the device obtained from UV-Vis-NI R spectrophotometry. b) Voltage dependence of percent relative luminance. At two extreme states, the device is absorptive with two different colors, blue and red. At intermediate voltages, the device possesses high luminance values, such as 94% at E=0.2V. Conclusions In conclusion, we have demonstrated that the PEDOT/PSS can be used as a transparent electrode material for both pa tterned and non-patterned electrodes as an excellent replacement for conventional ITO el ectrodes. Line patterned, interdigitated,

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113 PEDOT/PSS electrodes were subjected to the electrochemical deposition and electrochromic switching of PEDOT and PBEDOT-Cz. The major experimental drawback was high surface resistance of these electrodes which prevented homogenous electrochemical deposition. We have im proved conductivity by mixing PEDOT/PSS with high boiling point solvents and obtained surf ace resistivity values of as low as 600 /sq. We have constructed and characterized all-organic ECDs using these improved PEDOT/PSS electrodes. Two ECDs were utili zed to show how the all-polymer ECDs can yield different coloring phenomena. The first ECD achieved a 51% transmittance change at 540 nm upon switching and was highly stable with only a 5% cont rast loss after 32,000 switches. The second ECD demonstrated two distinct colors (blu e and red) at two extreme states with a transmissive intermediate state.

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114 CHAPTER 6 ECDS BASED ON PROCESSABLE DIOXYTHIOPHENE POLYMERS Electroactive and conducting polymers are potential elec trochromic materials, exhibiting diverse color variety and high contrast ratios, and as is especially illustrated by the work reported here, the desirable pro cessability of thermoplastic polymers. As synthetic organic chemistry allows the de rivitization of the monomer structure, controlling the electronic proper ties of the conjugated backb one allows fine-tuning of the EC properties. Recent focus has been placed on the application of solution processable electroactive polymers in ECDs due to thei r promising potential in the fabrication of large area devices.70, 140 Our group has developed a new family of alkyl substituted poly(3,4alkylenedioxythiophene) derivati ves due to their ease of synthesis, high chemical stabilities in the oxidized state, high optical contrast values, and fast switching between redox states.27, 29, 30, 144 Early studies have shown that PEDOT is a useful material for electrochromic devices, switching from dark bl ue to transparent s ky blue by applying a positive voltage.19, 145 By increasing the size of the al kylenedioxy ring, and by increasing the number and size of the substituents on the ring, the electrochromic properties, including switching times and c ontrast ratios are enhanced.29, 30 Dual polymer electrochromic devices have been fabricated and evaluated using PProDOT-(Me)2 as the cathodically coloring polymer. 77, 92, 146 PProDOT-(Me)2 is one of the most promising polymers for EC applic ations in the alkylenedioxythiophene based family, with a % T of 78% at max of the polymer (578 nm), a relative luminance

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115 contrast of 60% in the visible region and sub-second switching times.22, 30 Transmissive/absorptive EC devices have been fabr icated by sandwiching a gel electrolyte between two IT O coated glass slides, one in which PProDOT-(Me)2 has been electrochemically deposited and another in which a complementary anodically coloring polymer has been deposited, such as PBEDOT-NMeCz or PProDOP-NPrS.77 By applying a potential bias, the optical density of the device can be controlled from highly transmissive to deeply colored. As the PProDOT-(Me)2 is oxidized, changing from a dark purple to a transmissive s ky blue, the anodically colo ring polymer becomes reduced, switching from a dark blue to either a transmi ssive yellow or colorless. The devices have an overall colorimetrically measured lumina nce change of 60% and retain the optical response after tens of thousands of double potential steps. Dual polymer electrochromic devices have been fabricated which can modulate the reflectivity of electromagnetic radi ation from a surface. PProDOT-(Me)2 can be electrodeposited onto working electrodes consisting of slitted gold92 or gold coated porous membranes.26 The working front facing electrode is connected to a complementary polymer deposited onto a count er electrode hidden behind the EC active electrode. The reflectivity of the gold surface is observed as the cathodically coloring polymer switches from absorptive to transmissive with an applied potential. A device consisting of PProDOT-(Me)2 deposited on a top gold surf ace and another polymer deposited on the counter electrode gave a % R of 55% at 600 nm.92 The device also showed electrochromism in the NIR region by yielding a % R of 80% at 1.3-2.2 m. Recent synthetic work has involved placi ng sufficiently long al kyl chains on the alkylenedioxy ring to induce solubility in or der to chemically polymerize to yield an

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116 organic soluble and processabl e polymer. Many methods for chemical polymerization of thiophene derivatized monomers have been developed. Chen and Rieke147 and McCullough et al.148, 149 accomplished this by synthesizing and characterizing regioregular poly(3-alkylthiophen e)s followed by Iraqi and Barker,150 Guillerez and Bidan,151 and many others. The latest method developed by McCullough and coworkers has utilized a Grignard metathesis polymerization,152, 153 which does not require long reaction times, cryogenic temperatures, or nontri vial preparation of reagents often seen with other methods and affords the polymer in the neutral state. Our group has extended Grignard metathesis coupling to the synthesi s of soluble and pro cessable derivative of PProDOTs.37 In one example, dibutyl derivitiza tion allows the synthesis of polymers with adequate molecular weights and solubility in common organic solvents such as THF and toluene to allow high quality films to be obtained. The polymer is electroactive, exhibiting an electrochromic switch from deep red-purple in the neutral state to highly transmissive sky blue in the oxidized state. Our group is now developing a new series of alkyl and alkoxy substituted PProDOTs that are highly soluble in common organic solvents, exhibit high optical contrast, and possess sub-second switching times. Spray Coated Electrochromic Polymer Films Four disubstituted alkyl and alkoxy PProDOTs were sp ray cast onto ITO/Glass electrodes and the electrochemi cal and electrochromic properties of the polymer films were analyzed. Figure 6-1 depicts the chemical structures of these derivatized PProDOTs. PProDOT-R2 polymer films were sprayed from th eir 0.6% w/w toluene solutions using an airbrush. Highly homogenous films with thicknesses contro lled from 30-300 nm with RMS surface roughness values of 10-25 nm were attained as determined by profilometry.

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117 A photograph of spray cas t films of PProDOT-(CH2OEtHx)2 (left) and PProDOT-(C18)2 (right) on ITO coated glass slides is shown in Figure 6-2. S OO O O S OO S OOPProDOT(EtHx)2PProDOT(CH2OEtHx)2PProDOT(C18)2n n n S OO nPProDOT(Hx)2O O C18H37H37C18 Figure 6-1. Chemical structures of solution processable PProDOT-R2 polymers Optoelectronic Characterization All spray coated polymer films were high quality and homogenously covered the electrode surface (Figure 6-2). Their electr ochemistry was carried out in a 0.1M TBAPF6/propylene carbonate electrolyte solution using a Pt flag as a counter electrode and silver wire as a pseudo reference. Potentials were then calibrated against Fc/Fc+. Figure 6-3 (a) and (b) show cyc lic voltammograms of PProDOT-(Hx)2 and PProDOT(EtHx)2, respectively. Black curves show the firs t CV cycle of the as spray cast films. After this first cycle, the onset of oxi dations occurs at lower potentials (2nd cycle, red curves) and the subsequent cycles give th e same stable current response for multiple cycles. The shift in the onset potential is about 200 mV whic h suggests a 0.2 eV increase in HOMO level of the polymer films upon initia l switching. We have also investigated the changes that occur after the first electrochemical switching by comparing the optical spectra of as-sprayed an d previously redox switched neutral polymer films.

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118 2.5 cm Figure 6-2. Photograph of spra y cast films of PProDOT-(CH2OEtHx)2 (red, left) and PProDOT-(C18)2 (purple, right) from 0.6% w/ w toluene solutions using an air brush. The substrates used were two si zes of ITO coated glass slides (7 x 50 mm and 25 x 37.5 mm). Copper tape was a dhered to the top of the slides to ensure an electrical connection. Figure 6-3 (c) and (d) show the UV-Vis sp ectra of neutral films of PProDOT-(Hx)2 and PProDOT-(EtHx)2 where the black spectra repres ent the neutral and as-sprayed polymer films. Upon switching (oxidation fo llowed by reduction back to the initial neutral form), the spectra show a significan t red shift (red spectra) which results in a decrease in bandgap. This sh ift in both the ox idation potential and the band gap is attributed to a doping induced extension of th e polymer chains giving a longer effective conjugation length, a phenomenon observed with dibutyl deriva tive of PProDOT.37 The quinoid geometry of the oxidized state leads to an ordering of the chains.154 After reduction, the extension of the ch ains is partially preserved; leading to a larger effective conjugation length compared to the as-made spray cast films. Table 6-1 lists the major peaks in the UVVis spectra of polymer films as well as the optical band-gap taken for each film. Th e data for electrochemically polymerized films are shown for comparison. All polymer films exhibit a similar dark blue-purple color in the neutral state and are sky-blue transmissive when a sufficient potential is applied to fully oxidize the film. The optical bandgaps ranged from 1.8-2.0 eV for all polymers. The UV-Vis spectra of the polymers often gave three peaks as listed in

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119 -1.0-0.8-0.6-0.4- -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 I (mA/cm2)E (V) vs. Fc/Fc+ -0.6-0.4- -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 I (mA/cm2)E (V) vs. Fc/Fc+ (a) (b) 400500600700800 0.0 0.2 0.4 0.6 0.8 1.0 AbsWavelength (nm) 400500600700800 0.0 0.2 0.4 0.6 0.8 1.0 AbsWavelength (nm) (c) (d) Figure 6-3. Cyclic voltammograms and UV-Vis spectra of polymers PProDOT-(Hx)2 (a and c) and PProDOT-(EtHx)2 (b and d). Black CV curves: Initial redox switching of the as-sprayed films. Re d CV curves: Second and subsequent switching. Black spectra: As-sprayed polymer films. Red spectra: Previosly switched polymers. Table 6-1. Peaks (nm) and optical band-ga ps (eV) from the UV-Vis spectroscopy of PProDOT derivatives. PProDOT-R2 (echem) a Eg(echem b (film) c (redox) d Eg(redox) e PProDOT-(Bu)2 632, 573, 533 1.86 eV 544 576 1.84 eV PProDOT-(Hx)2 634, 584 1.80 eV 595, 553 575 1.84 eV PProDOT-(EtHx)2 623, 567, 517 1.96 eV 611, 559 618, 543, 521 1.92 eV PProDOT-(CH2OEtHx)2 609, 555, 518 1.87 eV 581, 543 600, 551 1.97 eV PProDOT (C18)2 NA NA 594, 553 590, 559 1.89 eV aElectrochemically polymerized films. bOptical bandgap of electrodeposited films. cSpray cast films from polymer solutions in toluene. dSpray cast films from toluene solution th at were switched from neutral to the oxidized state and back to the neutral state. eOptical bangap of a previously switched spray cast film.

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120 Table 6-1, but each spectrum differed in inte nsity and resolution. The fine structure is much more resolved for the polymers PProDOT-(EtHx)2 and PProDOT-(CH2OEtHx)2, which have branched substituents. PProDOT-(Hx)2 and PProDOT-(Bu)2 exhibit these peaks, but are more poorly resolved and the high energy peak exists only as a shoulder. A full spectroelectrochemical series of PProDOT-(Hx)2 was performed on a spray cast film in 50 mV increments as illustrated in Figure 6-4. As the working potential increases, the absorbance in the visible regi on decreases, as transitions at ~ 950 nm and ~ 1600 nm start to grow. At higher potentials, the -* transition continues to decrease as the 950 nm peak levels out and the 1600 nm peak continues to grow. These NIR peaks are attributed to charge carriers that form due to oxidation of the film.31 By removal of one electron, a polaron structure is formed, gi ving rise to the 950 and 1600 peaks. At high potentials, a second electron will be lost , forming a bipolaron structure which is associated with a second overlapping peak also at ~1600 nm. The overall effect of oxidizing the film is a bleaching of the dark blue-purple color to a more transmissive sky blue. Relative luminance studies (%Y) between fully reduced and oxidized states were performed as a function of pot ential on all polymers using the CIE 1931 Yxy color space. The voltage dependence of the relative luminance offers a perspective on the transmissivity of a material as it relates the human eye percepti on of transmittance over the entire visible spectrum as a function of doping on a single curve. The light source used is calibrated taking into account the sensitivity of the human eye to different wavelengths. Figure 6-5a compares the %Y ch ange of derivatized PProDOTfilms spray cast onto ITO coated glass electrodes. PProDOT-(Hx)2 and PProDOT-(C18)2 show a

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121 Figure 6-4. Three dimensional surface of the spec troelectrochemistry of a previously switched spray cast fi lm of PProDOT-(Hx)2 on an ITO coated glass slide. Spectra were taken at 50 mV increm ents between –0.4 to 0.5 V vs Fc/Fc.+ -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 20 30 40 50 60 70 80 90 Relative Luminance (%Y)E (V) vs. Fc/Fc+ PProDOT(Hx)2 PProDOT(EtHx)2 PProDOT(CH2OEtHx)2 PProDOT(C18) (a) (b) Figure 6-5. (a) Relative luminance ch ange (%Y) of spray cast PProDOT-R2 films. Red and black: linear substituents. Blue a nd green: branched substituents. (b) %Y vs. applied potential (circles and triangles) superimposed on cyclic voltammetry (solid line, 25 mV/s) for PProDOT-(EtHx)2.

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122 gradual change, beginning at 0.6 V continuing until saturation is reached at -0.4 V. PProDOT(CH2OEtHx)2 gives the sharpest %Y change where the 80% of the total change is obtained within 100 mV (red cu rve, between 0.2V and 0.1V). PProDOT(EtHx)2 also shows a sharp transition in the lumina nce as a function of potential similar to PProDOT-(CH2OEtHx)2, demonstrating the effects of linear vs. branched derivatives on electrochromic properties. Switching studies of spray cast and electrodeposited polymer films were performed by monitoring the %T at max as a function of time duri ng a potential step where the polymer oxidizes and reduces. The switching times , taken at 95% of the full switch, give subsecond times for all films w ith the exception of PProDOT-(C18)2. PProDOT-(C18)2 gives a much higher value at 2.2 seconds. Th e long alkyl chains lik ely decrease ion flux, leading to longer switching times . Table 6-2 lists the change in relative luminance and singular wavelength percent transmittance cont rast ratios along with the 95% switching times for these films. Electrochromic properties of the spray cast films are comparable to the electrodeposited films (Table 6-2, last three columns), which s uggests these polymers will prove useful for large area electrochromic devices where processing is important. By superimposing the CV and relative lu minance results, the relationship between the electrochemical properties a nd the optical properties of a f ilm are readily seen. As an example, Figure 6-5b shows a CV switching of a PProDOT-(EtHx)2 film between -0.5V and +0.4V at a scan rate of 25 mV/s. The %Y change of this film in the same potential window is also shown to demonstrate that optical changes accomp any electrochemical changes as the applied potential is increased/decreased stepwise.

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123 Table 6-2. Electrochromic prope rties of spray cast films. %Ya %Ta t(s)a %Yb %Tb t(s)b PProDOT-(Hx)2 53 62 0.70 73 49 0.43 PProDOT-(EtHx)2 65 78 0.80 41 53 0.50 PProDOT-(CH2OEtHx)2 56 80 0.57 46 48 0.60 PProDOT-(C18)2 49 51 2.2 N/A N/A N/A aSpray cast polymer films. bElectrohemically deposited polymer films Thickness Dependence of PProDOT-(EtHx)2 Films Electrochemical and optical properties of electrochro mic polymer films strongly depend on the thickness of the film used. In or der to obtain the best EC contrast with minimal switching time and power, judicious se lection of film thickness is necessary. Five spray cast films of PProDOT-(EtHx)2 were prepared with thickness values ranging from 28 nm to 236 nm to establish the thickn ess dependence of the EC properties. Figure 6-6a shows the cyclic voltammograms of these films obtained at a scan rate of 50 mV/s. As the deposited polymer layer gets thicker, PProDOT-(EtHx)2 gives a more broad redox process. At the same time, half-wave potential (E1/2) shifts from 0.05V (28 nm) to 0.13V (236 nm) vs. Fc/Fc+. Figure 6-6b shows the amount of charge passed through polymer films as a function of time when the films are switched from their neutral states to oxidized states with a large magnitude pot ential step (-0.8V to +0.8V vs. Fc/Fc+). As expected, it is much faster to fully switch a 28 nm film (0.4 s) compared to a 236 nm film (>2 s). However, in order to obtain a reasona ble EC contrast, a film thickness of more than 50 nm is required as shown in Figure 66c. It is important to note that, although the %T values of bleached states are high for all thicknesses (76% to 92%), it is the %T value of the colored state which subs tantially changes as the thickne ss is varied from 28 nm to 236 nm (4% to 58%). Finally, Figure 6-6d s hows the relationship be tween the switching charge (Qd), visible EC contrast (%T), and 95% switching time (tswitch) as a function of

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124 thickness. Depending on the application, f ilm thicknesses of 75 nm to 200 nm provide sufficient EC contrast with sub-second sw itching times and low power requirements. -0.6-0.4- -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 I (mA/cm2)E (V) vs. Fc/Fc+ 28 nm 62 nm 86 nm 150 nm 236 nm0. 0.0 0.3 0.6 0.9 1.2 1.5 1.8 Qd (mC/cm2)T (sec) (a) (b) 0 10 20 30 40 50 60 70 80 90 100 %TTime (sec)0501001502002500.0 0.2 0.4 0.6 0.8 1.0 1.2 30 40 50 60 70 80 0.2 0.4 0.6 0.8 1.0 1.2 1.4 050100150200250 t switch%T QdThickness (nm) (c) (d) Figure 6-6. Thickness dependence of electroch emical and electrochromic properties of spray cast films of PProDOT-(EtHx)2. Coloration Efficiency We have measured the coloration efficien cies of all spray ca st films by passing a low constant current leading to bleaching of the films over a period of 20 seconds. As an example, Figure 6-7 shows the change in the transmittance of a film of PProDOT(CH2OEtHx)2 along with the variation of its colora tion efficiency, as a function of the amount of charge passed during this slow bleaching. The value undergoes a maximum

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125 at ~2,680 cm2/C and follows a form similar to that observed previously on other PEDOT derivative ECDs.28 The other polymers had peak values ranging from 650-2350 cm2/C. Considering the physical si gnificance of the peak values, this can be taken as the coloration efficiency for the initial change during the electrochromic switch. A peak is observed as the electrode charges (e.g. doubl e layer) and the optical response lags the current slightly when Faradaic doping begi ns. Although the charging process is linear with time (constant current), the %T curve te nds to saturate and this results in lower values at higher doping levels. 500 1000 1500 2000 2500 3000 020406080100 20 30 40 50 60 70 80 Slow Coloration Efficiency (cm2/C)Passed Charge (mC/cm2)%T % Doping Level %T Figure 6-7. Slow coloration efficiency and percent transmittance as a function of passed charge for a 150 nm film of PProDOT-(CH2OEtHx)2. Electrochromic Devices In this work, we report the cons truction and characterization of absorptive/transmissive and reflective ECDs based on two of the a bove described soluble conjugated polymers, PProDOT-(CH2OC18H37)2 and PProDOT-(CH2OEtHx)2. These

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126 polymers were spray coated from toluene (0.6 % w/w) in their neutral (colored) states onto ITO coated glass substrates and gold coated polycarbonate membranes using an airbrush. After removal of so lvent by vacuum drying overnig ht, the resulting films of PProDOT-(CH2OC18H37)2 and PProDOT-(CH2OEtHx)2 were highly homogeneous and observed as blue and reddish purple, respectively. The aver age thicknesses of the films were around 150 nm with 20 nm surface r oughness as characterized by profilometry. Much like the other PProDOTs studied, th e diethylhexyloxy derivative, PProDOT(CH2OEtHx)2, exhibits a reddish purple color up on casting from solution. Once the film is fully oxidized and reduced back to the neutral colored state, the color changes to a blue/violet. Therefore, the band gap decrea ses in comparison to the randomly oriented cast film due to a longer e ffective conjugation length after redox switching. The original reddish-purple color was inaccessible afte r the first redox process. The polymer, PProDOT-(CH2OC18H37)2, has a blue/viole t color upon spray casting which does not change after redox switching. Two types of ECDs were constructed with these soluble polymeric EC materials to demonstrate their light modula tion characteristics in the vi sible and in the NIR region. The first type is an absorption/transmissi ve ECD that operates as an electrochromic window and the second type is a reflective EC D that comprises a cathodically coloring polymer deposited on a gold substrat e as the electrochromic layer. Absorptive/Transmissive ECDs ITO coated glass (20 /sq) was used as the substrate material. PProDOT(CH2OC18H37)2 and PProDOT-(CH2OEtHx)2 were used as the cathodically coloring polymer and PBEDOT-NMeCz as the anodicall y coloring polymer. Redox charges of the

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127 two complementary colored polymer films were matched using chronocoulometry to provide a balanced number of redox sites fo r switching by stepping the potential between +1.0 V and -1.0 V. Cathodically coloring polymer films (PProDOT-(CH2OC18H37)2 and PProDOT-(CH2OEtHx)2) were fully oxidized to their tr ansmissive blue state and the anodically coloring polymer (PBE DOT-NMeCz) was fully neutra lized to its transmissive yellow state. The films were then soaked w ith polymer gel electrolyte until the entire polymer surface was uniformly covered. ECDs were assembled by placing two EC polymer films (one doped and the other ne utral) facing one a nother separated by a polymer gel electrolyte layer as shown in Figure 6-8. Figure 6-8. Schematic representation of an absorptive/transmissive type PProDOTR2/PBEDOT-NMeCz device Relative luminance studies were performe d on the ECDs as a function of applied voltage using the CIE 1931 color space. Figur e 6-9 shows the percen t relative luminance change with respect to applied voltage of a PProDOT-(CH2OEtHx)2/PBEDOT-NMeCz device along with photographs of the extreme states. At nega tive voltages, the device is absorptive purple with a luminance value of 16%. At positive voltages, the luminance increases to 93%, yielding a 77% luminance co ntrast. As expected, this value is greater than that of the individual polymer films. In the device, EC polymers operate in a complimentary manner where both of the polym ers contribute to the absorption in the colored state. When the bias is switched, the absorption of the polymers in the visible Transmissive Substrate ITO PBEDOT-NMeCz PProDOT-R2 Polymer Gel Electrolyte

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128 region decreases and the device is observed to be transmissive yellow in the bleached state. It is important to not e that the redox charging/discharging of the polymer layers were balanced with respect to each other for optimal performance. Therefore, it is possible to access the extreme doped/ dedoped states. As for the PProDOT(CH2OC18H37)2/PBEDOT-NMeCz device, the relative luminance contrast was somewhat lower (45%), which is due to the lo wer optical contrast of PProDOT-(CH2OC18H37)2. Spectroelectrochemistry plays a key role in examining the optical changes that occur upon doping or dedoping of an ECD. Figur e 6-10a shows a series of UV-Vis-NIR absorbance spectra of PProDOT-(CH2OEtHx)2/PBEDOT-NMeCz ECD under various applied voltages. When a negative voltage (e.g. -1.0 V) is applied to the PProDOT(CH2OEtHx)2 layer, the polymer film has an absorptive violet color with the interband -* transition split into three distinct peaks at 520, 556 and 607 nm. This splitting was also observed for other PProDOT deri vatives which is likely due to vibronic coupling due to a high degree of regularity along the polymer backbone,155 through Davydov (exciton) splitting,156 and discrete polymer segment absorptions.157 At this voltage, the PBEDOTNMeCz layer is oxidized and exhibits an abso rptive blue color. When the voltage applied to the PProDOT-(CH2OEtHx)2 is increased stepwise to +0.6 V, this layer starts to oxidize and a new peak at ~980 nm, corresponding to polaron charge carriers of PProDOT(CH2OEtHx)2, starts to increase in intensity at the expense of the -* transition.158 At the same time, another absorption is induced at longer wavelengths (>1,100 nm), which is attributed to bipolar on charge carriers.159 Upon further oxidation (+1.0V), the PProDOT(CH2OEtHx)2 layer is completely oxidized atte nuating the absorption between 500-600 nm, changing the filmÂ’s color from deep violet to transmissive blue. At this voltage, the

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129 Figure 6-9. Voltage dependence of th e relative luminance of a PProDOT(CH2OEtHx)2/PBEDOT-NMeCz device and photogr aphs of the device in the bleached and dark states. In the colo red and bleached states, the device exhibits a relative luminance of 16 %, and 93 %, respectively resulting in a 77 % luminance contrast. PBEDOT-NMeCz layer is reduced, changing from blue to a transmissive yellow. This results in the bleaching of the device, changi ng from violet to transm issive yellow-green. The absorption spectra of the device in the two extreme states (colored and bleached), extracted from the spectroelectrochemical seri es for clarity purposes, are shown in Figure 6-10b. It is important here to note th at the optical density (O.D.) of the -* transition of the PProDOT-(CH2OEtHx)2 layer (max = 607 nm) is much higher than that of the PBEDOT-NMeCz layer (max = 418 nm). Therefore the PProDOT-(CH2OEtHx)2 layer has a greater optical contribution to the device. This is an ex pected result of the charge optimization process carried out during the de vice construction. Prior to device assembly, we individually measured the redox switc hing charges required to fully switch the polymer layers for different polymerizati on charges. We found that less PBEDOT-

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130 NMeCz is required to match the amount of redox charge contained in PProDOT(CH2OEtHx)2 for a full switch; thus we are able to use a lower amount of PBEDOTNMeCz in order to optimize the exchanged charge. As a result of this, the PBEDOTNMeCz layer has lower O.D. values. Figure 610b, curve a, also clearly explains the lightly absorbing green-toned color observed in the bleached state. The neutral PBEDOTNMeCz (max at 418 nm) is slightly yellow and, co mbined with the residual absorption of PProDOT-(CH2OEtHx)2 around 600 nm, prevents the de vice switching to a completely colorless bleached state at +1.0 V. While not addressed in this chapter on the soluble PProDOT based devices, using a high gap al kylated derivative of PProDOP overcomes much of this residual color in the bleached state. (a) (b) Figure 6-10. (a) Spectroelect rochemistry of a PProDOT-(CH2OEtHex)2/PBEDOTNMeCz device as a function of applied vo ltage (a) +0.8, (b) +0.5, (c) +0.4, (d) +0.3, (e) +0.2, (f) +0.1, (g) 0.0, (h) -0.1 , (i) -0.2, (j) -0.5 V. (b) Spectra from the two extreme states of the device. The composite coloration efficiency is a useful term for measuring the power efficiency of a device since it determines the amount of optical density change (OD) per injected/ejected electronic charge (Qd) during a large magnit ude redox potential step. Figure 6-11a shows both the opti cal density (solid line) and charge accumulation (dashed

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131 line) change as a function of time while the device is stepped between the absorbing, colored state and the transmissive, bleached st ate. Contrast ratios are calculated by the transmittance change between the colored and the bleached state. A nearly complete 95% of the full switch is reached faster (0.3 sec) for the PProDOT-(CH2OEtHx)2 device than that of PProDOT-(CH2OC18H37)2 device (4.6 sec). Our grou p previously reported the values of ECDs containing derivatives of E DOT as the cathodically coloring polymer and PBEDOT-NMeCz as the anodically co loring polymer to be between 400 cm2/C and 1500 cm2/C.130 Table 6-3 shows the composite coloration efficiency values of the two absorptive/transmissive ECDs consisting of soluble PProDOT derivatives as the cathodically coloring layer. As an example, wh en 95% of the total contrast is obtained, the of PProDOT-(CH2OEtHx)2/PBEDOT-NMeCz device was 4804 cm2/C at 609 nm while that of PProDOT-(CH2OC18H37)2/PBEDOT-NMeCz device was 1294 cm2/C at 595 nm. Coloration efficiency () is directly related to the cont rast ratios in organic polymers, as well as the reciprocal of the injected charge. The higher of the PProDOT(CH2OEtHx)2/PBEDOT-NMeCz device is due to th e faster switching rates and more abrupt optical changes in a narrow potential window of the PProDOT-(CH2OEtHx)2 polymer. A high can provide a large optical modulati on with small charge injection or extraction and is a crucial parameter for practical ECDs. We have also measured the coloration efficiency of a PProDOT-(CH2OEtHx)2/ PBEDOT-NMeCz device by passing a low cu rrent through the device galvanostatically leading to bleaching of the device over a period of 20 seconds. Figure 6-11b shows the change in the transmittance of the device, along with the variati on of the coloration efficiency, as a function of the amount of charge passed during this slow bleaching.

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132 Table 6-3. Optical and electrochemical data for coloration efficiency measurements % of full switch T tox (s) Qd (mC/cm2) Da (cm2/C)b PProDOT-(CH2OEtHx)2 / PBEDOT-NMeCzc 100 50.9 2 0.34 1.37 4022 95 48.4 0.3 0.28 1.35 4804 90 45.8 0.19 0.26 1.33 5229 85 43.3 0.17 0.25 1.31 5251 80 40.7 0.15 0.24 1.28 5428 PProDOT-(CH2OC18H37)2 / PBEDOT-NMeCzd 100 49.2 6.00 1.06 1.27 1197 95 46.7 4.63 0.96 1.25 1294 90 44.3 4.01 0.92 1.22 1332 85 41.8 3.63 0.86 1.20 1398 80 39.4 3.31 0.82 1.18 1435 a . D = log (Tox/Tred) b . = D / Qd c. max = 609 nm, Electrode area = 5.5 cm2, %Tred = 2.2%, % Tox = 53.1%, T 50.9 . d . max = 595 nm, Electrode area = 5.0 cm2, %Tred = 2.8%, % Tox = 52%, T 49.2 The overall %T changes for the slow switc hing of this device are similar to those observed with the large potential step sw itching experiment in Figure 6-11a. The value undergoes a maximum at ~3,850 cm2/C. Although the charging process is linear with time (constant current), the %T curve tends to saturate and this results in lower values at higher doping levels. When 95% of the full contrast change is reached using this slow experiment, the device is operating w ith a coloration efficiency of 1550 cm2/C. This is lower than the 4804 cm2/C obtained at 95% of full cont rast for the large magnitude potential step experiment due to the non-Faradaic curr ents involved in the slow doping/dedoping of the polymers that causes a higher amount of total charge (0.6 mC/cm2, Figure 6-11b) needed for complete bleach ing. Using a large magnitude potential step eliminates this current; hence the device requires less charge to switch (< 0.4 mC/cm2, Figure 6-11a).

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133 Reflective ECDs A reflective ECD constructed as shown sche matically in Figure 6-12a and utilizing an organic soluble PProDOT-(CH2OEtHx)2 as the surface active layer has been investigated. The conducting polymer switc hes between a transmitting (conducting) and absorbing (insulating) state upon a reversible redox switch ca using the device to be gold 051015202530 0 10 20 30 40 50 60 95%, CE = 4804 cm2/C 90% CE = 5229 cm2/C 80% CE = 5428 cm2/C %TTime (sec)0.00 0.10 0.20 0.30 0.40 Q (mC/cm2) 1000 1500 2000 2500 3000 3500 4000 (cm2/C)Passed Charge (mC/cm2)0 10 20 30 40 50 60 70 %T (a) (b) Figure 6-11. (a) Chronoabsorptometry (solid line) and chronocoulometry (dashed line) for a PProDOT-(CH2OEtHx)2/PBEDOT-NMeCz electrochromic device monitored at = 609 nm as the voltage is ste pped between the colored state (-1.0 V) and the bleached stat e (+1.0 V). Device area = 5.5 cm2. (b) Variation of the coloration efficiency () and %T as a function of the charge passed as the device is bleached slowly at a co nstant current value of 0.03 mA/cm2. reflective and magenta absorptive, respectivel y. The polymer was deposited onto a goldcoated porous polycarbonate membrane (act ive layer electrode) and onto a gold-coated Kapton (counter electrode) by spray coating. Following the redox conditioning of the polymer films, the counter electrode polymer layer was fully neutra lized while the active polymer was fully oxidized to ensure a charge balance prior to device construction. The reflective devices were built in a sandwich configuration with the gold-coated membrane spray coated with PProDOT-(CH2OEtHx)2 shown in Figure 6-12b facing outward to allow a convenient reflec tive mode characterization.90, 91 The outward facing

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134 electroactive polymer is responsible for th e surface reflectivity modulation whereas the counter electrode polymer only contributes to balance the electroactive sites and its optical properties are not observed and do not affect the device operation. The underlying counter electrode was constructe d using the same polymer to attain electrochemical compatability with the visible outward faci ng electrode. The reflective type electrochromic device was assembled us ing a high viscosity polymeric electrolyte composed of TBAPF6 dissolved in a PMMA matrix swollen by acetonitrile/propylene carbonate. Figure 6-13 shows a full set of reflect ance spectra for a device as a function of applied voltage. When a nega tive voltage is applied to th e active layer, the spectrum exhibits the lowest reflectance (%Rmin) at 600 nm, due to the absorption from the -* transition which gives rise to the color of active layer (reddish purple) and a high reflectivity in the near infrared from 800n m to 2000 nm. By applying a positive bias to the active layer, the polymer oxidizes and the -* transition attenuates with an increase of charge carrier transitions in the NIR region. At this voltage, the polymer active layer is transparent, so the gold layer dominates the reflectance. The device exhibits a reflectance contrast (%R) value of greater than 55% in the visible region (=600 nm), and a %R of 75% in the NIR (=2000 nm).

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135 PProDOT-(CH2OEtHx)2Active layer PProDOT-(CH2OEtHx)2Porous substrate (PC) Gel electrolyte Porous separator Gold/Kapton Gold layer Neutral stateOxidized state(a) (b) Figure 6-12. (a) Schematic device structur e of a reflective ECD. (b) Photographs exhibiting the neutral and the oxidize d appearance of the active layer on a gold reflective surface. 400600800100012001400160018002000 0 20 40 60 80 100 +0.80 V %RWavelength (nm) -0.50 V -0.15 V -0.10 V -0.06 V -0.04 V -0.02 V 0.0 V Figure 6-13. Spectroelectroc hemistry of PProDOT-(CH2OEtHex)2 containing reflective device as a function of applied voltage.

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136 An unusual electrochromic switching pr operty was observed as the device was switched between –1 V and 0 V as illustrated by Figure 6-14a. The reflectance contrast at 2000 nm was greater than 70 %, while at 609 nm; the %R was less than 3%. The result is the creation of an ECD that is IR active while undergoing no visible color change. This unexpected behavior is eas ily observed by spectroelectrochemistry where the two UVVis-NIR spectra of this device were compar ed at –0.80 V and –0.02 V as shown in Figure 6-14b. Despite a large contrast in the NIR region, the spec tra are quite similar in the visible region. As shown in Figure 6-14c, when a voltage is applied to the active layer between +1.2 V and +0.05 V, there is essentially no change in the NIR reflectance contrast at wavelengths longer than 1600 nm while there is substant ial color change as evident by the response in the visible region. We speculate that this asymmetric switching of the ECD can be attributed to the full penetration of the NIR light through the neutral polymer layer, as opposed to the strong absorption of the visible light (Figure 6-14b, -0.8 V). Therefore, when the polymer is partially oxidized (-0.8 V to –0.02 V), the reflected NIR light is more sensitive to the optical changes that occur upon oxidative dopin g close to the electr ode surface than the reflected visible light. As a result, we observe the absorption of the charge carriers in the NIR region (%R decreases) with minimal depletion of the* absorption in the visible region. This does not occur for the spectroelec trochemical series of the same polymer in the transmissive mode, since only the transmitted light that goes completely through the polymer layer is detected. Similar findings have been addressed previously in the literature.81, 86, 160

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137 Figure 6-14. (a) Electrochromic switching at = 2000 nm and = 609 nm as the voltage of a PProDOT-(CH2OEtHex)2 containing reflective device is stepped between -1.0 V and 0.0 V. (b) Reflective spectra of the same device at -0.8 V and -0.02 V illustrating the NIR electrochromism with minimal color change. (c) Spectral response of the device between +1.2 V and + 0.05 V. Conclusions In conclusion, soluble and processable di alkyl and dialkyloxy substituted PProDOT derivatives are promising candidates for EC D applications due to their synthetic versatility, fast switching times, and hi gh coloration efficiencies. Spray coating techniques allow homogenous deposition of polymer films over large and irregular surfaces on various substrates such as ITO a nd gold. This solution processing of polymer films introduces the possibility of construc ting large area ECDs a nd patterned devices. Absorption/transmission ECDs built from these spray-coated films have the greatest coloration efficiency values reported to date (4804 cm2/C at 95% of the full contrast), and

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138 high luminance contrasts (77 %) with less th an 1 second switching times. The judicious selection of polymers along with ingenious engineering will further improve the switching rates of these devices to make them promising candidates for dynamic display applications. Reflective ECDs built with su rface-active polymer layers using porous substrates prove to be superi or light modulators both in th e visible and the NIR region. The state of the art assembly of the ECD re veals the best switching properties of the solution processed EC polymer films. The ability to modulate the NIR absorption/reflection by controlling the appl ied voltage without any noticeable color change in the visible region is an ongoing research interest and an unusual phenomenon in polymeric ECD systems. Overall Summary and Perspective The last few years have seen an immense set of developments in the properties of EC polymers and their application to multiple device configurations. Using a variable monomer structure, or by adjusting the composition of copolymers, composites, and blends, the varied color stat es of EC polymers can be t uned across the visible spectrum and applied to wavelengths outside of the visible. The facts that an EC polymer presents a minimum of two distinct co lor states, can be continuously modulated as a function of the applied voltage, and in some instances can present multiple distinct color states offer a significant amount of flexibility for displa y and window type device s. As solvent and electrolyte swollen films that ar e directly attached to transparent conducting or reflective metallic surfaces, the electroactive centers are directly addressable and can be switched quite rapidly. Using appropr iate electrode geometries, rapidly switching polymers with sub-second response times can be envisioned. Considering that the accessible switching speeds with perceptible contrast values ar e now down to approximately a tenth of a

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139 second (44% change in 0.16 seconds, see Figur e 3-5b), it is reasonable to compare this rapid switching rate to video rates of ~15-30 frames-per-second (fps). As the redox states are set by applied potential and are physically separated from one another, polymer-based ECDs can present a significant level of elect rochromic memory. Initial studies described in Chapter 3 showed that with a small refr eshing pulse, polymer based ECDs can retain their colored states with esp ecially long resting periods (m any minute) at open circuit. The repeated switching stabilities m easured, typically of the order 104 to 105 deep cycles, provide an area where improvement is desired. As these limitations are likely due to loss of electroactivity and/or adhesion at the elec trode surface, methods to make the electrode materials more compatible (e.g. surface functio nalization) with the EC polymer structure are being implemented. Recently, Lu et al. demonstrated switching stability values of 106 cycles for ECDs using ionic liqui ds as the electrolyte material.58 These viscous, yet highly conducting salts are molten at room temperature for easy processing, are environmentally stable, and they can be opera ted at a broader voltage window compared to that of the conventional salts. Back-side electrical contacts method describe d in Chapter 3 is a new approach to address porous electrodes as opposed to the conventional fron t-side contacts. Back-side contacts eliminate the unattractive traces on a display device by hiding them on the back of the electrodes without any conductivity loss . Ultimate application of this method will be for vertically integrated, 3-D configured devices where the space for circuitry lines is extremely limited. By utilizing solution or melt processable conductors on porous electrodes, region specific contacts can be made to promote conductivity on desired areas.

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140 As demonstrated in this dissertation, patterning is the key to create complex electrode structures for fast switching and multi-color ECDs. For the low-end applications, optical lithography is far too co mplex to be cost effective which makes is less accessible to chemists and material scie ntists. Soft lithographic techniques such as microcontact printing (CP) offer promising resolution values with relatively simple processing steps. It is envisioned that the CP will enable selective deposition of EC polymers on substrate surfaces with suprem e precision and independent addressing of active sites. Line patterning, another non-lithographic techni que, eliminates the mask preparation steps by direct printing of the negative of a desired pattern using a commercial printer. This opens up possibili ties for inexpensive “throwaway” devices without compromising performance. Using the micropatterned IDEs de scribed in Chapter 4, it is possible to “average” the colors of a number of EC polymers to obtain new color states. The lateral configuration of th e anode and cathode on a single surface with minimal separation also eliminates the need for an additional electr ode. An interesting potential feature is to bring this technol ogy to a smaller size le vel with the goal of constructing a set of micro-pixels independe ntly addressable in order to be more applicable to multi-color displays. PEDOT/PSS, the only commercially avai lable conducting polymer solution, is extensively used in this work for both cons tructions of truly allorganic ECDs and line patterned organic electrodes. It proved useful as a transparent electrode material since the surface resistivity is low enough for elect rochemical deposition and switching of polymers and its conductivity does not change in the potential window the ECDs operate. By combining the versatility of line pattern ing technique and the processability of

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141 PEDOT/PSS, patterned organic electrodes ar e easily obtained. Thicker coatings of PEDOT/PSS yield lower surface resistivity va lues, however this also decreases the transparency of the electr ode. We have optimized the PEDOT/PSS thickness to be 300 nm (%T 75% in the visible region) to yield 600 /sq of surface resistivity. Any improvement to increase the bulk conductivity will greatly contribute to the competence of these organic electrodes and the field will encounter a greater amount of device applications using PEDOT/PSS (or anothe r conducting polymer) as the electrode material. Solution processability of EC polymers is a major advantage for construction of large area devices such as smart windows. Cathodically coloring PXDOTs have been successfully solubilized and the spray co ating method proved useful for homogenous deposition of these polymers on transparent and reflective electrodes. In order to construct window type ECDs from all-proce ssable polymer parts, a soluble anodically coloring polymer needs to be developed. As such, two electrochemically compatible, complementary colored polymers can be depos ited on two transparen t conductors to be used for window type ECDs. Another future enhancement is to adapt these polymer solutions as inks to be used for patterning via inkjet printing. The most crucial part of this technology is the physical properties of the ink such as the viscosity and the surface tension.

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151 BIOGRAPHICAL SKETCH Avni A. Argun was born on June 9, 1978 in Ac payam, Turkey, where he spent his childhood until he finished elementary school in 1988. After attending Nazilli Anatolian High School for 7 years, he began his undergra duate studies in the fall of 1995 at Bilkent University, Department of Chemistry in Ankara, Turkey. With an intense physical chemistry education and a special interest in polymer chemistry, he came to the University of Florida, Department of Ch emistry in the fall of 1999 to begin doctoral studies under the supervision of Professor J ohn R. Reynolds in the area of patterning of conducting polymers. His professional career as a Ph.D. will begin as a post-doctoral fellow with Professor Paul Holloway at the University of Florida, Department of Materials Science and Engineering.